Single-phase 16A? That's the standard? My tiny apartment homelab probably uses at least a third of that (I should measure its consumption actually...).

Said 50m² apartment has 16A triple-phase installed, but the breaker box is wired to be capable of safely drawing up to 63A triple-phase from the grid if I needed it.

Yep. You can obviously upgrade to 4.5 and 6 kW (still single phase, I think), or go three phase. Single house probably do that. But I'd be surprised if the apartments had more than 3 kW installed.

This results in a big payoff for efficiency, however: high efficiency AC and heat pumps for water go a long way in increasing how much you can do on so much little power.

Do Italians not use electric kettles?

Only those that consume 3 kW max :)

Jokes aside, I think that this ends up incentivizing an efficient use of energy. That's not so bad after all.

I mean sure, but there are certain appliances(like kettles) which really can't get any more efficient, so if you want to boil some water......3kW overall limit for the entire apartment is seriously limiting. Like, if someone is boiling water and another person uses a hair dryer, it will blow fuses?

My place was renovated about decade ago and has 3x25A... And I don't have anything special like Sauna and circuits are pretty much original...