W.r.t the DB box
This is why get an electrician is better.. gen con

1. No 10mA RCD for water heater is against regulations.

Electricity Regulations 1994
Regulation 36. Protection against earth leakage current.

For an installation in a place where the floor is likely or where the wall or enclosure is of low electrical resistance, protection against earth leakage current shall be afforded for any final circuit supplying electricity to any equipment, either individually or in a group, by a residual current device having a rated residual operating current not exceeding 10mA.

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2. Priorities in the wrong place (w.r.t wire sizing for AC and WH).

WH current model (instant heater) 3600W Pana/Hitachi, 3800W Toshiba/Rinnai. Easily 16Anominal running current. Only 2.5mm cable for such heavy load device? ST recommends 4mm.
WH current model (storage tank) 3000W Joven. Also not a light load device.

Aircon current model only use like 5-7A nominal current (1-2hp) need 4mm cable?
It has peak load only at the point when motor start but if inverter (VFD) model the current draw is the lowest among all starting method.
3. For the 100mA whole house RCD, its a waste of money putting that in (alongside a 30mA whole house RCD)

100mA is the legally maximum allowed tolerance for the fixed apparatus (lights AC and fan) portion, meaning, instead of using two blocks 30mA and a 100mA, the single 30mA can easily do the job to cover the entire house circuit.
30mA for 13A socket safe. But what if you got distracted or someone turn on the switch by mistake when you change a lightbulb? That is on 100mA...

The 100mA is NOT a required "must put in parallel item". Historically back then (20-30 years back) people all use flourescent tube, non-inverter AC, non-inverter fridge. These (back then) tend to trip sensitive RCDs because they were rather lossy devices.

Nowadays all use very efficient almost lossless LED, no point risking it for 100mA, as 30mA whole house RCD safer (in case any appliance anywhere faulty you still can buy me a coffee).
100mA RCD cannot and will not save human life in fault condition. It only protect against fire risk. >50mA bye bye.
Only 30mA RCD can save human life. Its a very common thing to use 30mA in US, UK, AU especially for schools and residential installation.

According to diagram
1. WH circuit not follow standard (missing 10mA RCD)
Fix: put in 10mA RCD (statutory requirement for wet floor circuit)

2. Heavy load wire smaller than light load?
Meaning to say..if its already like that n didnt hack (stuck with it);
But if already redoing wiring (insofar as relocate DB =redo wiring) might as well do it right.

3. The RCD higher delta (higher mA), is like sleeping security guard function.
1 sharp and alert guard is all you need.
No point to hire 1 sharp and alert and hire one more "sleeping on the job" although the "JD" allows them to sleep on the job while sitting at their post. You pay for the sleeping guard cost.