Resilio could work, but the directories it uses by default are outside of the safe zone on SteamOS, an update could wipe them or break them in various ways. A few people have put together scripts to reconfigure the default directories, as well as scripts to run after an update, but I'd rather not rely on that.

Syncthing works out of the box if installed as a flatpak, due to SteamOS using flathub as the default app store, and keeping all those directories in the safe zone that doesn't get touched by updates.