A router that seems to get "fixed" temporarily after power-cycling likely has a small amount of RAM that is being consumed very quickly, which causes all functions to slow down over time. You can try disabling as many unnecessary logging functions as possible, but I'm inclined to believe this won't yield much improvement.
If it were just the wireless portion having the issue, I'd suspect wireless interference from neighbourhood signals. While you've yet to show us the wireless survey results that were asked for previously, I suspect that those results would be inconsequential.
In essence, you'll benefit from a new router, preferably something from the WiFi 5 generation or better.