Distribution cabinet - What size is reasonable and recommended?

not just the usual 2 circuits per room

It is now common to have 1 circuit for 2 rooms. No separate circuit for lighting.

But I see it this way too: If there is space available, better to go one size bigger. If you have network cabling, a cabinet is useful where you can also install a patch panel, router, etc. ("complete cabinet with multimedia panel")

One size bigger. Either 4-slot or, as already mentioned, 3-slot with [EHZ] (ask the network operator!) With a heat pump, a ripple control receiver may be required, which also needs to be installed somewhere. I also don't see any [APZ] in the example!

Classic example with me: electrician planned 3-slot (2 [eHZ], 1 [APZ]), ripple control receiver ([RSE]) as a DIN rail device and got approval from the network operator. Network operator during commissioning attempt: [eHZ] ....WHooot? Network operator later: Whoooot, heat pump? [RSE] missing!!!!1!elf. Where is the 3-point pickup from the last war?

End of the story: [APZ] removed, 1 slot replaced with 3-point and not enough TEs in the 3rd slot.

Therefore: if possible, go for a 4-slot cabinet :)

Or you simply connect the heat pump to the household electricity, save the second meter, and then have no blackout periods anymore.

A heat load calculation for the distribution cabinet provides further insights. I will now also have to deal with this topic.