Change whole box also not too expensive lah. Just get the parts following the existing + add 10mA RCD or RCBO to WH circuits.
Yours is single phase (not 3 phase).

1x 63A main MCB - 2P (2pole) 6kA RM51.60
https://shopee.com.my/%F0%9F%94%A5READY-STO...3048.6462998954

1x ABB RCD 63A 2P 30ma (should be made in italy) RM129.00
https://shopee.com.my/ABB-RCCB-ELCB-RCD-40A...1048.7350447835

According to ST guideline they actually specify 100+30mA two device but for me, take one unit is cheaper as the 30mA can do everything the 100mA does without compromise life protection. ONE unit main 30mA 'bao whole house' will give life protection.
Don't listen to nonsense bullshit about 30mA always trip. Some dinosaur faux electrician still want to use 100mA or 300mA so they dont need to pick up calls. These (100 or 300) are useless for life protection (imagine u tertouch live circuit when changing bulb because someone knocked the switch live, or toaster leak electricity due to faulty device). More than 50mA confirm RIP.

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I run mine and parents' place on 30mA. What good is RCD if not sensitive enough for life protection.

many pcs at least follow existing layout now - MCB each ABB MCB 1P/6kA - for the individual C16 buy C16 as necessary how many units according to current DB layout RM7.40/each
https://shopee.com.my/ABB-MCB-1P-6kA(6A-10A...4526.6860538286
This one if you want to split the shared circuit in your DB box you can buy more according to cable size.

1x each per WH circuit -Schneider 10mA RCBO (this one just replace inline aka swap the MCB for WH circuit also can) RM100.00/each PER WH
For Water Heater Circuit should trace back that individual neutral for it and use 10mA RCD OR RCBO per water heater.
ST guideline say according to the law you need 10mA for Water Heater or wet floor places' socket.  to prevent this from occurring to you: https://www.nst.com.my/news/nation/2020/08/...e-electrocution

10mA RCD need to add with the MCB. Usually for ABB 10mA RCD this ard RM170.
10mA RCBO already include the MCB inside it so you can discount off
https://shopee.com.my/SCHNEIDER-RCBO-25a-2p...1048.2774414891

Lastly change the busbar to fresh one.. 23-way like RM9.20 only
https://shopee.com.my/(READY-STOCK)-1-PCS-M...6841.5064694032
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Above proposed is mostly ABB parts except the WH 10mA RCBO.
Schneider stuff also can use or Hager also can. Mostly the sensitive 10mA one is expensive. Rest of components about same price for top tier brands. Then find electrician supply labour to change all the parts (parts you can source yourself better coz usually they propose cheapo brand that is not much better)

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To speculate why ppl die from faulty water heater
because if the main RCD is those 100 or 300mA.. this only protect for FIRE protection and not LIFE.
so main RCD cannot save you if its >50mA (almost common to see all these 100/300 useless RCD in residential). Yours also 100mA.
and then the WH circuit does NOT have 10mA (high sensitivity) RCD according to ST guideline means NOTHING is protecting LIFE.

If you have WH 10mA on WH, and a main 30mA for whole house, LIFE is protected on TWO RCD devices. So even if one fail (murphys law) you still have a backup.
Still can come back and buy me a coffee for saving your LIFE.

edited

the spd here is type1 for direct strike. i cant find the datasheet on this.
you may need to install a fuse to limit any current leak

you will need additional SPD type 2 to limit the voltage after a strike. most equipment wont survive a volt leak from a type 1 SPD
consider
type 1 = outdoor water filter. you wont drink from this tap
type 2 = clean filter. can shower
type 3 = cleanest can drink

spd is like water filter
type 1 handle the large direct strike current. thats why its written there as 10/350
however, the leakage also high. means if kena strike there will be some voltage pass through the filter

thus the need of type 2 spd for 8/20 wave
those belkins/mrdiy are type 3
for water heater you need 10mA
for socket outlet is 30mA
lamps can be 100mA

Yes can direct replace the 0.1A (100mA) with 0.03 (30mA). Yours is single phase, so the main RCD it will be a 2P(oles) 63A 30mA type. This will be a direct replacement (same physical size as currently installed type)..The really fat one (RCD) is 4P (3P+N) for 3 phase wiring.

Good news, 2P RCD is still cheaper compared to 4P RCD.

For the MCB problem, you can get a same rating one as current (if faulty is C16 buy another C16 from ABB Hager Schneider to replace first).
All things AFTER main switch will be isolated as long as the main switch is off and verified with test pen.
Test pen also need to verify it lights up first, so as not to have false negative due to broken test pen.

Rule of thumb is when working with electricity, ISOLATE and TEST, TEST, TEST before touching anything.
Yes 10mA (0.01A) is the correct rating for WH RCD. Even more importantly for countries on 230V. For life protection I only trust the top tier brands. ABB / Hager 10mA come from EU.

Most water heater tend to be ELCB based (the one could be RCD but not sure how quality it is).. there are cheap RCD out there but for 10mA i would strongly recommend a top tier brand (ABB hager schneider); schneider RCBO just over 100 only; the rest standalone about 160-170/ea. Per ST guidelines, proper standalone RCD is still recommended.

You mentioned "Yes can direct replace the 0.1A (100mA) with 0.03 (30mA)"....do you means i can replace the "hager 63A/100mA that i have in my current set-up?
Yes, replace this with 30mA main RCD (specifically 63A 2P 30mA type)

I was thinking to trace the maxguard MCB C16/C20 that attached to the WH, replace it with branded MCB with 10mA or 30mA....will it served the protection purpose? MCB and RCD operate differently (like seatbelt and reverse sensor of your car).
MCB protect against current overload (FIRE risk from cable overheating and the copper wont melt instantly but its the plastic insulation that catch on fire and so on that cause house fire).
RCD principle it operate when there is any current imbalance(whatever input current must = output current, this is normal condition. If 10A go in but 8A come out means the 2A must have got lost somewhere.. and that is what will trip RCD..  with a 10mA RCD >0.01A loss will trip. with a 30mA >0.03A loss will trip. That lost somewhere means a fault already.. whether leak here leak there leak thru human body is irrelevant..to the RCD current cannot "lost" one..same as bank teller if cash in drawer not tally with system means they need to hunt and trace every sen as any error means something wrong somewhere.

Just like seatbelt and reverse sensor both protect but they protect diff thing and operate differently, same goes to MCB vs RCD.

For RCD, it constantly compare the IN - OUT current all the time and if there is more than the acceptable loss (based on RCD sensitivity 10mA 30mA etc) it will trip.
So because it need IN vs OUT (Live vs Neutral) comparison to work, the (IN) Live one is pretty easy to determine since u off the MCB 1 by 1 to trace, and the one that (when MCB off) doesnt turn on the WH, that should be the Live WH cable.
For the (OUT) Neutral, in DB box all the black wires typically connected to the common shared neutral busbar. This one is more time consuming as u need to isolate one by one each N (take out one by one) until you find the one that doesn't operate the WH, that should be the N WH cable.
Of course do a double take and mark these (L WH) and (N WH) with tape or sticker. Then plug them back in and confirm WH works now. Successfully identified the WH L and N now.

These (WH Live) and (WH Neutral) will need to be connected into the same 10mA RCD (load side).
The (power feed) L and N on the 10mA RCD Line side, can be connected to Line L busbar and N common busbar respectively.

To answer your question.. since already open up the DB might as well replace the whole strip of MCB and RCD.
The RCDs are more pricey than a whole strip of 6ka MCBs.
Adding 10mA RCD will need two slot more space though.

Btw, i will buy the parts and ask technician to help me to replace the MCB. 

Any ideals what is the different for the maxguard MCB C16 and C20?
One trip at 16A rating, one trip at 20A rating. MCB Sizing should be based on load and cable size. Usually it should be no bigger than C20 for 2.5mm (13A socket/WH) cable, and C10 for light circuit (1.5mm) but if its dedicated light circuit putting C6 also won't harm anything (extra safety) since your lights in totality for whole house won't exceed 1000W whereby a C6 for light circuit is way more than enough already.

Yes, you are right, the RCD 10mA should be installed for WH, it was regulated in 1994.... but not many people is aware of this!
Safety of electrical installation is taken too lightly in MY. Same old story, people maybe unaware, every year also come out in the news people die from WH electrocution. Even their 13A plug can be left hanging with the LNE inner wiring exposed for few cm instead of properly fixing up the plug to grip on the outer insulation
Checked my WH menu, there is a built-in RCD, but no specific on the spec of the RCD, not sure if it is 30mA or 100mA?
Seems vague to me. In the diagram it shows a RCD at the DB box Err on the safe side, this is cheaper (price of RCBO/RCD from top tier brands range from 100-170) than life insurance premium.

For MAINS RCD change to 30mA Bao-Kah-Liao whole house
Yes, I did this before when refreshing my 1995 DB box.
Scenario: For mains RCD change to 30mA (current ABB/Hager models selling now) they have anti nuisance trip; I think Schneider also has it.

My personal experience in parents' house: It was on 1995 era ABB 40A 4P 300mA before this when we took over as 2nd owner. Now using ABB 40A 4P 30mA, no complains.
Result:  No difference in trip problem or trip frequency for main 30mA RCD.

CONSIDERATIONS WHEN ADDING 10mA into OLDER HOMES with NO 10mA RCD
The thing about 10mA WH RCD/RCBO it will not work on shared neutral (10mA will always trip unless wholly dedicated L and N).
Just like company claim should only be for your own usage, you should not take receipts from next door diff company workers to claim into your claim submission, will trip finance_RCD

I face this problem before here, (my wiring was very messy as this is used apartment, previous owner do a lot of funny things, I even suspect the WH is also connected to ex-13A point or looped somewhere in the neutral)
Adding SPD and splitting shared MCB (2-3 wire into one MCB ) required more physical spacing.
End up expanded whole DB box to 36way also not enough (previously was falling apart elcheapo plastic box)
Then realised needed more than that so spill over to second DB box. 3 phase big modules really take up a lot of ways.

Solution was to run direct LNE cable from DB box to WH (so its always dedicated neutral), perfect.

CONSIDERATION WHEN REPLACING EXISTING (QUESTIONABLE CHINA MADE) 10mA RCD in NEWER HOMES
No issue just plug and play as WH Live and WH Neutral should already be on dedicated L and N.
Also easier since wires are identified already, just need to see which is load side and which is line side.
And for this (my own place) I went with big DB box (45 way) straight away to fit all the expansion upgrades like Type2 SPD and have enough room for everything (RCDs, MCBs, main switch, SPD)