I had it initially setup to run on Wi-Fi too, battery or charging.
Then I had my battery drain to 30-40% during afternoons, when I’m used to reaching evenings above 60%. Check app usage on settings: Syncthing.
Since I use it mostly for backing up photos, I found it better to enable it only when charging.
If you’re interested in Lineage, just check their device page and filter for set top box:
https://wiki.lineageos.org/devices/