I am affected by the same issue, I think.
I have not had any time to troubleshoot the problem yet but my system becomes inaccessible within one or two days and I have to cycle power to get back in.
It has been like this since I first set it up in June.
The WiFi going to sleep sounds like a good place for me to start looking for the problem.
I think I will bring it all back inside and leave the Pi plugged into a monitor and keyboard until it goes offline again and then check what is not working.