Ok, that was fast. My Fritz!Box keeps the channel mostly for weeks, at least for days, but interference of course will differ from place to place.
AVM Support told me that the reason for the box not simply going back to the fixed channel after a while would be that the box would still detect the interference there, but I can always just fix the channel again, the same one as before, and it will be stable again for a while.
Wi-Fi being complicated tech as it is, I can just live with something like this if I can make it work in some fashion, but I always wonder where this all is headed going forth. Well 
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If you can live without 5 GHz, and if the 2.4 GHz range isn’t too overcrowded with other networks at your place, it should help.
Ok, so the general App category doesn’t seem to be it.
I have a bit of occasional Signal use and I even have Discord (yeah, I know) running in the background, doesn’t seem to be a problem for me.
Android can be booted into a safe mode, which can confirm whether Apps installed by the user are the cause for an issue (if the issue is gone in safe mode) or not (if the issue persists in safe mode).
It isn’t as intrusive as a factory reset, but it might still be a bit of a pain with App login credentials and some other setup stuff, resources …
https://support.fairphone.com/hc/en-us/articles/360048843571