Time Machine in the "Cleaning Up..." state forever
I noticed that cleaning up old backups can be very slow. In my case, I can hear my external backup drive rattling along, so I know it's doing something.
You can check on progress by opening the latest backup log. Look for the directory with a name ending in .inProgress on your backup drive. Within it, open the log file with the most recent timestamp:
$ sudo less +F &quot;/Volumes/MacBackup/Backups.backupdb/MacBook Pro/2020-08-05-163227.inProgress/.Backup.618330747.626060.log&quot;

Substitute MacBackup, MacBook Pro and the backup/log path. If you're not familiar with less, press Ctrl-C, followed by q to exit.
The log file will tell you how much space needs to be cleared and which of the backups it's (very slowly) deleting. Looking at the amount of free space on your backup disk should give you an idea of how far along it is.