What is the most efficient 3-phase to single-phase AC transformer for a home grid
The 230/400V three phase system used in most of Europe can supply either three-phase 400V loads or single phase 230V loads (or a combination of both) with no transformer or special switchgear required, beyond standard switchgear such as MCBs.

230V single phase loads are connected between neutral and any phase (ideally, they should be divided up roughly equally between all three).

400V three phase loads, with or without neutral, are connected to all three phases, and the neutral if required by the load.

Any electrician worth their salt will be able to do this easily. If they can't, they should have no place anywhere near a three-phase board.