When adding your FreedomBox as a device in other Syncthing peers, set its address like "tcp://njoseph.me" instead of "dynamic". This allows your Syncthing peers to directly connect to your FreedomBox avoiding the need for relays. It also allows for fast on-demand syncing if you don't want to keep Syncthing running all the time on your mobile devices.
More tips and tricks on my wiki
https://njoseph.me/mediawiki/FreedomBox/Tips_and_Tricks
Documentation
https://docs.syncthing.net/users/config.html#device-element