I have been using Syncthing fork since before the official app was discontinued, and can vouch for its quality. My favorite feature is that it allows conditional pausing of folders based on phone state, such as if the phone is charging or connected to WiFi. Just be warned that the version on Google Play was no longer updated last time I checked (Googles fault), so you're better downloading releases from the Github repo.

Are there any issues or tricks for syncthing in termux, for e.g. always running in the background?