Commissioning of the network installation

Switch would be the cleanest; but very bulky. And it needs power.

However, if you are satisfied with 100mbit, you can also use so-called doopler. The 8 wires are basically distributed over 2 connections.

100 MBit only needs 4.

You can also "patch" telephone via the network. We have the TAE in the utility room, from there I go to the patch panel, from there somewhere else (specifically: hallway), there stands the Fritzbox, the second network cable then goes back to the utility room/patch panel.

Great. That's exactly how I want to do it too (even though the house isn't built yet )
In the utility room there will be the DSL/Telekom/TAE... whatever... connection.

All rooms (and kitchen) should get double sockets. The Fritzbox, however, should be located quite centrally in the living room (especially for WLAN and DECT). So this means utility room telephone cable to patch panel -> double socket living room Fritzbox -> back to utility room -> switch/patch panel -> star-shaped to the other rooms.

I still have a question:
Since it is a townhouse (3 floors in total): How do I reliably distribute the WLAN and DECT across all floors? Another Fritzbox on each floor? Or WLAN/DECT repeaters? This is still somewhat a mystery to me...

DECT will not cause any problems. WLAN you solve with an access point on every floor e.g.

no simple access points wired at strategic locations.

That means to the double sockets you have in the house...there you have to sacrifice a connection for the access point here and there...

Damn, Tomtom was faster...

Here in the apartment I have a WLAN repeater with its own SSID. Is that also necessary for an access point or does it have the same SSID as the "main WLAN" because it is wired?

Thanks first of all for the quick answers =)

You can do as you please... in the private sphere it is usually irrelevant