Regardless of what exactly is causing the problem it might be that as
long as you reboot once a day the problem never occurs. So rather than
wait for a slowdown reboot the router each night and see if the problem
remains.

If this does solve the problem you will be reasonably confident it is a
problem with your router and can either buy a new one or just continue
rebooting it once a day.

Some of them allow you to schedule regular reboots. I think an old Netgear router that I had allowed this. I had some issues (can remember what) so I scheduled a weekly reboot.

The Draytek 2862 that I am using seems solid, although I dont use wireless through it, thats handled by a separate BT mesh system.

Alan