not sure what syncthing does! I'm not a big fan of the cloud, I just knew of that app because The Linux Experiment channel talked about it
I used to have problems and visual glitches with text in flatpaks (like VS code, or Spotify, the text would be bugged), but as of the latest drivers that is fixed.
From my experience on Fedora, my advice is that after a new kernel installs, don't restart immediately, give it a minute or two to build the nvidia drivers for that kernel. If you are signing them for secure boot i guess the recommended wait is 5 minutes.
You can use this app to sync with proton drive https://flathub.org/apps/com.hunterwittenborn.Celeste
I'm even more excited about the Ticket Booth app, because even if/when I use streaming services, if I like a show I'll end up torrenting for archival purposes.
I don't trust that this content will be available on these platforms forever, so it really means a lot for me to have such a nice app to keep track of what I watched / liked / want to watch