I also did the same as you but because they discontinued the official syncthing app on Android I skipped the "sync to PC first step. Now I directly go to Borg from phone night time only when charging, the automate app is able to invoke termux and run the backup script

Many times it happened the syncthing app crashed in background and I didn’t notice before after several days, now if the Borg repository server (borgwarehouse, it’s a must have) doesn’t see activity after some user specified time sends me a warning email