I know this is unsolicited advice, so apologies for that, but did you consider that the issue could also be your server hard drive failing? I guess not a possibility if its really only recently synced pictures. Just wanted to throw that in.
Regarding backing up photos, Immich works well for that but is also a lot more than just photo backup, so maybe not a preference for you.
Checkout 3rd party jellyfin android apps. Findroid is working pretty well. Theres another one called Streamyfin which is catching up and a third one called Fladder, which is maybe a bit too early in development.