I did have a very similar experience to you ! I made this observation a while ago. Pictures synchronization issue in eDrive - #39 by aibd
Of course the sync process is a work in progress, so individuals syncing now will experience a pattern or errors different from some no so old reports.
In hindsight, I knew I backed up the things I “needed”, but if I had had a more robust backup earlier on, it would have been a good idea to delete my eCloud account at the point I thought I actually knew what was wrong and before I added a second device !