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 Just be warned that the version on Google Play was no longer updated last time I checked (Googles fault), so you're better downloading releases from the Github repo.
The sync would stall and I'd have to go retry, or it would fail with no error clear message. In the end, I had no idea what had really synched correctly. The app was unhappy if I deleted a photo too quickly.
With Syncthing, I sync to a directory that my Nextcloud user can access (a read-only mount), so I can still easily share photos using Nextcloud, for example.
(although it's unfortunate that the Android syncthing app is being retired. h/t for the heads up and the recommended alternatives)
I used Nextcloud sync in the past, but found it unreliable.