I have Syncthing installed via pacman (since flatpaks cannot keep a daemon running). For every game that I care about, I find its save file, move it to a sync dir, and symlink it back to where it is expected. My savegame sync folder has folders for the many varied places that games like to hide their saves - “gamedir”, local, locallow, “my documents”, dot_config, etc. The most fun part is finding out where the appropriate proton prefix is.

If I was starting over again, there’s a decky loader plugin that looks promising.

By exclusively playing on Steam Deck there is no need to sync anything ;)