Specific to your case, retrofitting will be required.

1. Need to check whether the existing LNE lines from DB box to that point in the shower (currently the round pin socket) is of sufficient wire gauge (2.5mm is bare min). Best advise (to fool proof it for next 30 years - run a new set of LNE cables from DB to the WH point in bathroom in 4.0mm Mega Kabel - its cheap if you do exposed/surface conduit).

1.1 Assuming 2.5mm existing cables, you can only buy the 3kW class heater, do not buy a 4800W one, it will catch fire.
1.2 At the bathroom last mile (where the round pin socket goes currently) - use a proper connector like the Centon water heater connector. Because ST regulation say must be direct connection - not socket. Loose connections cause fires.

2.0 At the DB box - retrofit in a quality 10mA RCD or RCBO, per each water heater. This is your life insurance for water heaters.

3.0 At the DB box - change the mains RCD (the one for whole house) to 30mA. This is 2nd line of defence. In case the 10mA failed to trip - the 30mA will catch it and trip (like a fail safe). If your 10mA failed, and whole house one is 100mA or 300mA which I suspect is the case - there is no backup - GG.com because >50mA leak thru human body, sayonara.

Why so many upgrades?
RCD also wears out after 30 years.. springs get weak etc.. aiya even original 1985 proton saga also need to change many things by now in 2022 lor.

Actually ST requirements are 100mA for lighting and 30mA for plug points (power points for equipment easily accessible by hand). In this case, 100mA main RCDs are acceptable, provided that you have 30mA to cover the plug points in addition to the main RCD. If there is only main RCD it has to be a maximum of 30mA, but ST doesn't really recommend 1 RCD for the whole DB.

Of course, this is in addition to water heater circuits which are required to be protected by 10mA RCD (dedicated).
Adding a 10mA RCD in cascade with a 30mA/100mA is not an issue. The device is meant to trip the power due to safety reasons, what may happen is that both may trip at the same time if there is a problem, which is more preferable to it not tripping when the problem occurs.

Doing it "flat" is of course preferable but will involve alot of work at the DB, which is dangerous if not done properly.
Both may still trip, due to tolerances of the device itself especially when 10mA and 30mA are quite close in rating. There is a proper way to do discrimination of tripping for the devices, but even without the discrimination it's still preferable to put them in cascade rather than not having sufficient protection.

Physical installation of electrical circuits can get quite misleading. If the cables are terminated in a daisy chain as per your picture, that is a parallel(flat) installation instead of cascading. Because the looping is happening at input of RCD only.

For example in below case the RCD are separate and will only trip 1 RCD if a fault happens at A / B / C / D respectively.


But if cascaded like below any fault in B may trip both RCD, but a fault at A will only trip the bottom RCD.


It's considered cascaded when output of the RCD is connected to the input of the next RCD, as you can see in the picture above.

In your case, if all 4 RCD are installed in a cascade, it's not dangerous but likely to be redundant because any fault at the end of the RCD will likely trip all of them, unless there is sufficient discrimination between them. RCDs are protective devices, so they don't introduce danger through their operation. But if RCD is not installed or is faulty, then the lack of protection from the RCD can be a hazard instead.

Actually devices that can cause safety hazards like water heaters already come with built in RCD nowadays, so it's not necessary to cascade multiple RCDs (as long as all DB circuits are properly protected already with the correct ratings).
Rather, it's recommended (and also more important) to test the RCD regularly instead to ensure that protection is intact. Some water heaters even allow you to test their RCD.
Because like what ceo684 has mentioned, the RCD will fail someday, and the only way to find out is by testing them.

Not many - its common in retrofitted and new build in SG
But in MY.. new builds got put but mainly China brands to save cost
Rarely you will get Schneider nowadays even for half million dollar place
30mA RCD, whole house (sum of all L - sum of all N return = acceptable losses)
Branches out from here to a bunch of little MCBs.
The 10mA is just for WH's own single L and single N. Input=output = acceptable losses and won't trip. IF the WH N is shared that means:
1) Installation is wrong in the first place
2) Useless for purpose of RCD, forever trip.
3) Most likely because it is not a dedicated LNE run from DB box to the WH unit.

Impossible they will trip each other off unless the WH circuit is shared neutral.
Correct.
And if there's a leakage of 20mA in a socket or light outlet unrelated to WH circuit, the 10mA will not trip.
No, this is acceptable.
Mana ada WH with built-in RCD.
ELCB got lah.

RCD is god tier protection (think of it as modern airbags)
ELCB is antiquated retired era stuff because "what if leakage don't leak nicely through Earth" aka leak elsewhere e.g. humans = don't t