Earlier quoted context omitted.
My device is not rooted, but I use Syncthing as well. I mainly sync my photos and my TOTP tokens (Aegis). The rest I don't care about. I used Nextcloud sync in the past, but found it unreliable.
Synchthing on Android is End of life, alas https://forum.syncthing.net/t/discontinuing-syncthing-androi...
Two alternatives exist:
1. Syncthing-Fork (https://github.com/Catfriend1/syncthing-android) - Works just like the official app. Install from F-Droid or GitHub. Single developer but active.
2. Syncthing in Termux - Run the actual Syncthing program in Termux. Takes more setup but uses the standard version.
The fork is easier to use, while Termux needs more setup but gives you the standard Syncthing experience.