4 Fritzboxes??? Or just Fritz Repeaters/APs? 4 boxes are totally pointless. You have to disable everything except WiFi on the others. Or do you operate all 4 as routers? Fritzbox+APs on LAN is also a good solution. Also easy to administer if it's Fritz. Which APs you use depends on the requirements, budget... You can't just switch manufacturers with mesh.
One connects to the Telekom modem, is located in the basement, and brings "the internet" into the switch. The guest LAN is on LAN port 1. One is on the upper floor, handling DECT and MESH WIFI there, with printers, PCs, etc. connected (since a double socket isn't enough for all the devices). One is on the ground floor, handling only MESH WIFI and DECT. One is in the tenant apartment, connected by cable (sockets patched accordingly) to LAN port 1 of the main FB. This way, the tenant apartment has internet + WLAN, but only guest network.
APs do not belong in the hallway area but where the most speed is needed.
I would say it depends. Our hallway is centrally located in the middle of the house, so it reaches every room optimally. If your hallway is only at the house entrance and, for example, the living room is central, that would probably be the best place! But what do you even want with maximum speed? WhatsApp and an occasional YouTube video will be enough, for more, you don’t really need Wi-Fi anyway.
Has any of you tested the network cabling with devices after the electrician's rough installation? That is, before the cables disappear under the screed.
Since the cables are to be laid interchangeably in an installation pipe, no. If anything should be wrong, the electrician must be able to replace them even after the screed is in. Otherwise, he has not done his work according to the standard.
Yes, they are supposed to go through empty conduits. However, when I asked the electrician if it would be possible to pull other cables through or replace them later, he said rather not. Therefore, I understood that the empty conduits are more intended as protection.