Commissioning of the network installation

I wrote further above: You only get true roaming with devices that cost around 400-500 euros and up.

I have three APs (I count the FritzBox as an access point) on the three floors and all have the same SSID, same WPA2 key, and otherwise – if possible, since unfortunately they are different devices – all settings are the same.

Thus, roaming for the poor already works with a tiny interruption. Only towards the basement I always have problems, but that is probably due to the FritzBox :-/.

Oh yes, each access point must have its own fixed radio channel, otherwise the handover does not work very well.

I will implement it that way as well. Here in the apartment, I am currently using a Wi-Fi repeater (also 2.4 and 5 GHz). However, the router in the living room and the repeater in the hallway have different SSIDs. This way, I can see what the device is connected to or can force the connection.

...and you only have half the network power left, but the main thing is to push ahead

Because it is a repeater (:
In the house, I use a corresponding access point

2) Fritzbox on the ground floor, DSL comes from the telephone socket in the basement, LAN is laid back to the basement via cable. Additional access point on the first floor.

This is the better solution for me.

How do you want to handle the handover between APs with WLAN? Meaning you go upstairs and are seamlessly handed over to another access point?

I have an access point on the upper floor (FRITZ!WLAN Repeater 450E, plugged into the socket and connected with a short LAN cable to the adjacent LAN port). Since I don’t fiddle with my phone while going up the stairs, I can’t say if the handover is seamless :P. At least we have "seamless" WLAN on the upper floor.

Disclaimer: The thing is called a repeater but can be operated as an access point.