OMG, 100x Downlights!?
Usually I will isolate the area and just focus on the number of circuits for the lighting.
Eg, Master Bedroom- 1x Cove Lighting, 3 or 4 circuits for Downlight/Track Lights. Any other lighting will be just looping.
Same goes for power socket and I usually use the 2-gang instead of the 1-gang SSO.

It is also best to use different ELCB/RCCB for power and lighting circuit. For my own DB, I used 3 different RCCB, 1 each for Power/A/Cond (100mA), Lighting (30mA) and my Solar Heater (10mA) for practicality and safety.
Storage tank heater uses 3KW heating element while instant shower heater uses 3.6KW heating element.
In terms of efficiency, nothing beats Solar Heater.

Rule of thumb, cable size is equivalent to 10x of current carrying capacity, eg 2.5mm = 25A.
Unless you are talking about 3HP and above AC, there is no need to go for 4mm cable size.

In the market there are 2 types of DB that either:

1. MCB/Isolator as Main Incoming
2. MCCB as Main Incoming

For better protection and easier wiring go for the option 2 though it costs more initially.
Local brands like Safety/MaxGuard/EPS all have these DB with 2 type of incoming as option.

100mA for power circuits are usually good as one will not need to worry about nuisance trip. As all power circuits are connected to a single 100mA, there is a build up of leakages. Hence putting a 30mA into the power circuit is too sensitive.
Most lighting especially LEDs are very efficient hence the chances of earth leakages is very minor hence a 30mA is best to protect it. Main point here is should a trip occur in the middle of the night on the power circuit, one would not need to wander around in the dark or relying on torch lights. I also forgotten to mention that I specifically told the electrician to wire my fans to the lighting circuit. In the event the trip in the power circuit cannot be resolve at least I still have my fans running.
MCCB does come in TPN configurations as well. At any given time I will always choose a MCCB over an isolator (costs aside).

If the electrician mentioned 2 different RCCB, then it is 1 each for Lighting and Power Circuit. If you have Heater (instant/storage/solar) it is best to have an additional RCCB of 10mA.

3-Gang SSO is rare, best to use 2x 2-Gang so it is easier to find in the market.