Planning of the technical room (distance between devices)

We are getting a smart home, so it's not really comparable... But otherwise, there are eHz meters that some providers use; with those, 2 meters fit in one row. 3 meters is just A LOT. 3 conventional meters of this design are about 75cm. We are planning to have only one meter with heat pump and photovoltaics, which would save 2 rows/50cm width in your case. Saving fuses, etc., is partly difficult. Regarding the switch, if the connections of the Fritzbox are not sufficient. We will try to connect as much as possible with cables and then add 1-x additional access points, for example in the upper floor, basement, etc., where we will probably need significantly more.

That is a good point, but I don't quite understand why you want to put a switch between the router and the end devices again. Usually, every router can handle that out of the box. Or do you mean because of the possibly too many end devices (if the router only has 4 ports)?

With 4 ports on the router, you don't get very far. That's why, in a typical average installation, you usually have a 16 or 24 port switch.

I installed an 8-port. Then the two from the router, and that is enough for us. A small network cabinet next to the house connection/breaker box was sufficient.

Yes, exactly something like a network cabinet with 6-9 height units and 10" width, available from €50. It can neatly accommodate 1-2 patch panels (each with 12 ports to be terminated), a switch, Fritzbox, a small NAS, Hue base station or similar, possible POE injector for a camera or other small items.

do you have a picture of that on hand?

Alright, I took a picture.
The technical room looks very chaotic at the moment, as I am in the process of replacing our old server with a new one.