Like new install on every restart

Are you getting prompted to configure a new user account? If so, then the /var/db/.AppleSetupDone file is being deleted which triggers Setup Assistant to run again.

If you are not getting prompted to create a new user account, then those questions may be due to the system thinking an OS update was just performed, but not remembering your previous answers. I would think it could be a corrupt .plist.

Are you by any chance running any anti-virus apps, cleaning/optimizer apps, or third party security software? If so, then they are most likely causing your problem since they interfere with the normal operation of macOS. Uninstall those apps by following the developers' instructions since these types of apps are not needed on a Mac & usually causes more problems than they solve. You can try booting into Safe Mode to see if that allows the system to remember you have already answered these questions.

There are so many .plists.:(

To find out if it's system wide or user specific, try this as it will create new user level plists...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

@BDAqua has you covered.

When you "reinstalled the OS a while back", was it due to this problem? Or another issue? That reason may still be the cause of the problems here.

And did you reinstall macOS over top of itself or by performing a clean install where you first erased the disk?

If a clean install, did you test the clean install before restoring from a backup?

Did you try Safe Mode and did you have the issue there? You need to boot into Safe Mode, then reboot back into Safe Mode again to make sure. Then see what happens when you reboot normally.

If you want to try to troubleshoot your main user account, you may need to compare items between the main user account and the new user account to see which file may control this behavior. Maybe first start looking into the list of Login Items (doubtful if Safe Mode did not make a difference). There may be some other way macOS ends up setting up the trigger to provide that walkthrough of questions & notes after an OS update. I have no idea what you may be looking for here. I tried a quick online search, but came up empty. It could be as simple as a hidden file being placed somewhere within the home user folder, or perhaps the trigger is located elsewhere. I'm not what options macOS has for trigger such events during login, perhaps there is a special file with a list of users meant to receive the update & this user is not being removed from the list.

You may also want to try running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if the First Aid summary says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If errors are listed, then run First Aid again until the errors are gone. If after several scans the errors remain, then run First Aid while booted from Recovery Mode.

You may just find it easier to try restoring the user account from a backup. I'm not sure of the best way to do this so I would leave it to others to advise you on this option.

ave you run Disk Utility's Disk First Aid on all connected drives?

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

You can use Drive DX to possibly get a better view of Drive health…

https://binaryfruit.com/drivedx