I'm bringing this to public as discussed in PM.
Yes, you can force a full factory flash. Yes, the flash-all script and the Android Flash Tool both do this - they flash bootloader, radio, and all OS partitions. If you want to be really
thorough, you can check the "Force flash all partitions
" checkbox, this also requires a data wipe.
I'm not sure why system apps would remain disabled following a data wipe.
The Google One backup isn't able to back up everything. It cannot back up application data, and it does not back up all system settings.
Regarding the screen, I've noticed the opposite with my Pixel 5. I have "Stay Awake" turned off, but despite having my screen timeout set for 1 minute, it turns off and locks after about 15 seconds.
Battery...Not sure what to tell you.
Consumer electronics are cantankerous beasts indeed and fraught with gremlins.