Time Machine backups should live on Apple Filesystems. The Ext4 or btrfs in Synology's is a timebomb for Apple's proprietary data blobs. It has something to do with the filesystem attributes.

I asked my father to just give up on it. Just manage pictures through the file system. Photos (former iPhotos) databases also get corrupt on a Synology NAS. It's a matter of time. The strange thing is that it can go well for months on end, giving you a false sense of security.

Do iCloud or local drive backups or stay away from Time Machine and Apple Databases is my advice (although my father had a lot of problems as well with an "incorrectly unplugged" external HFS+ drive). I can't find the sources right now but after my father's last drama (and there have been several) I did some intensive searching and this was my conclusion.

I did use time machine on external USB HDD and after some time it got corrupted. Happened multiple times over the years. Then I tried Time Machine using AFP and SMB on remote NAS. Those also got corrupted multiple times.

The lesson I learned was - do not use TimeMachine.

Now I have been using Vorta (UI for Borg) https://github.com/borgbase/vorta for a long time and everything is fine.

Same. I really like it. Also sending my backups to https://borgbase.com which was relatively painless to setup.