Hi guys

New to the forum but I've lurked a lot. Have gotten quite a bit of really good advice from this forum.
I'll state off the bat I'm a DIYer. I've studied the regulations quite in depth and I'm very serious about safety.

I live in an apartment in Cape Town. And as is usual for many Cape Town apartments, my incoming supply is 3 phase.

When I moved in the panel was a mixture of really old breakers and new breakers.
Some problems I had right off the bat:

Kitchen plugs weren't on a phase with an earth leakage (ouchie).
Only some of my plugs were on an earth leakage, everything else was not.
It became apparent quickly why this was the case, one set of plugs and the lights had a earth/neutral fault, so obviously as soon as I installed a EL everything would trip immediately. Took a really long time to find where it was shorting.

I've rewired the apartment as:
Main breaker (3p + N) -> EL (3p + N) -> Everything else (wired from the 3 phase EL)

Finally my question:
My incoming supply has a 35amp breaker.
Now I've noticed every other apartment has a 40a incoming supply breaker or 50a.

How common is a 35a incoming supply breaker?

I want to remove my geyser (takes a LOT of space) and replace it with an instant water heater, but it is rated at 21kW, which with 35amp breaker leaves virtually no wiggle room.

My thinking is the main supply breaker was replaced by the previous installer with whatever he had.

I obviously don't just want to upgrade the breaker to 40a, I simply don't feel it is safe.
So what I'm wondering is, how would one go about determining if the breaker can be upgraded, who should do it, etc?