Time Machine couldn’t complete the backup

Certainly worth the time to plow through all this, but I didn't find an answer to my specific situation. I think I discovered what triggered the issue, however. I changed the scheme on my machine to use iCloud to become the primary document storage for this two-user machine (two users, two separate logins, etc.). It looks like Time Machine is tripping up over files that once resided in the machine but have now been punted over to iCloud. I was successful is creating a new backup destination on my original AirPort Time Capsule, so far at least (creating the new backup is going to take about 18 hours or so). But at least that'll give me some reassurance that I have backed up the M2 Mini before I start experimenting with finding out what files that Time Machine is choking on, and why. Maybe.

Fixed it, after finding the offending files in the Deleted Users portion. After deleting the entire folder, Trash refused to empty, reporting that three of the files in Deleted Users were still in use (they obviously weren't). I was able to force empty the trash using the Terminal command sudo rm -rf (followed by a space), not hitting [Return], and then selecting everything in the trash and dragging that to the Terminal window. Then I hit [Return] and that did the trick. Time Machine then did several backups, both manually and automatically on the hour, and I was able to scroll back through the Time Machine backups and select and inspect files back from current day/time and then all the way back to 2023 and random accesses in between. It looks like all is well. Thanks for the support!