Backend quota is close to being exceeded

Not sure of the question. Without any advanced options there is no log file. You have to give it the path.
What you say is correct if you mean the --log-file-log-level default so this means you need the verbose.

Right. Depends on what you want. If it helps, you might have it thin out versions to avoid filling up again, however any file that exists only between versions that something like custom retention policy kept are unavailable for restore because no remaining version got it. There are lots of retention options possible.

You have to if you want an immediate deletion. You don’t if you want slow deletion as new backups omit it and old backups age away. When the last version having the file is deleted by retention policy, file is gone.

You can easily have File Explorer look for changes if you like. If the only thing there is programs that could easily be reinstalled somehow, there’s not much point backing it up at all. You want to backup YOUR data. You’re correct that in some cases versions build up. If I put datemodified:this month in the search box then look at results, I see the VirtualBox update I just installed. If I switch view to Details and select all files then right-click and get Properties, I see it’s got about a GB of data. Longer period would certainly be more.