I used to use Dropsync for Dropbox as well, so I was looking for a similar solution.
Turns out the easiest way is to enable webdav on the seafile server.
Then use foldersync for Android, that way you can sync dedicated folders to your phone.
I use this as well to not only backup, but also sync my pictures taken with my phone.
That way I can delete pictures on my laptop if I don’t like them and they will be deleted on the phone as well, very convenient.
With Seadav, you can also place the file of your password manager (I use SafeInCloud) in a separate library of your seafile cloud and let the password manager sync using webdav.
I tried Synchronize Ultimate as well, but I always had problems with the software…
It is far more powerful than foldersync, but unreliable, sometimes the scheduled syncs dont run so I stayed with foldersync. Perhaps other people are more lucky
Sry for my late reply and thx for the suggestions @res80 and @bionade24 - unfortunately I only use Apps from f-droid.org, neither foldersync nor Synchronize Ultimate is there. Furthermore I didn’t find any FLOSS app fitting…