Had a pixel 8, and now a new pixel 9a. I think the problem is actually a bit messy. On my house I have several access points. There is a chance when syncthing is working, and I am going up or down, phone changes the access point. Syncthing possibly gets a ' oppsie, didn't finish that! Let's go for the next one' kind of issue. Of course, never looked into logs or anything so this is just pure speculation.
Usually the kind of corruption on the photos is the kind that beginning is always there, but at some point gets replaced with gray, hence my theory about the files.
From what I tried, next cloud just has so much clutter that makes the syncing part a bit of a slog. If that was rock solid with OK speeds, that would be nice. But there is somich stuff with it that I don't care and tries to enforce (like users and user groups) that I find just too much