FRITZ!Box 4040

I would consider an old PC with OPNSense because it's funny to experiment with it (if you don't need wifi), or else you can get a FRITZ!Box (talking as an Italian). I had a 4040 that FRITZ!OS bricked somehow (bootlooping with the original OS and running fine with OpenWRT) and I installed on it OpenWRT. Supported FRITZ!Box are pretty easy to mod, you need a python script and the image, and obviously a bit of patience because you need to edit the python script.

GenieACS works fine with fritzboxes. What exactly does not work for you? How do you feed the ACS URL to your boxes, do they even register in the ACS?

i had horrible experience with the axe5400. went to fritzbox, never looking back

Used quite a bit by regional ISPs in Ireland

AMV Fritz.box. all you need to connect your stuff. Most is WiFi anyways, these days.