Satellite distribution in network cabinet

The Fritzbox 6690 also requires quite a bit of space due to its upright design

Nothing speaks against using it lying down as well.

One more note: If you use the Fritz as a WLAN access point: installing it in a server cabinet dampens the signal!

For the 6690, I have also already found 19-inch mounting kits – just search for "FRITZ!Box 6590 / 6591 / 6690 Cable Rackmount Kit 19 inch"

Proud price – but then you have it neat ;)

Yes, that is really a proud price. I don't think it's worth it. Yes, I know the Wi-Fi will be dampened, but in the basement that's not so important. Upstairs come the repeaters that are connected via LAN bridge. I still don't know which HE to take for the cabinet. What would you say, 9 or 12? You said 7 HE could be tight, especially in the future.

Well, your listed equipment is:

1x Patch panel 24 ports with Keystone mount
1x 16 port switch
1x Fritzbox 6660 or 6690
1x power strip

each 1 RU, so a total of 4 RU. To what extent you want to expand in the future is up to you and you should choose the size accordingly. :)