You’re very close. Your nose is touching the finish line.

If you’re using option user 'syncthing' in the OpenWrt Syncthing conf (/etc/config/syncthing) on your ext4 formatted USB drive and the permissions (perms) for /mnt/mmcblk0/syncthing read syncthin syncthin using ls -l /mnt/mmcblk0/syncthing then you’re all set.

To be absolutely sure use chown -R syncthing:syncthing /mnt/mmcblk0/syncthing to ensure all files, folders within are converted in their permissions to the syncthing user.

Go into the Syncthing GUI, delete ay & all existing folders you may have ATM. Then go into Syncthing GUI → Actions → Settings & set the default path for all new folders to /mnt/mmcblk0/syncthing :

Restart Syncthing by rebooting the device or just try Synchting GUI → Actions → Restart.

Don’t create new files/folder via SSH/SFTP in the /mnt/mmcblk0/syncthing dir. Any that are are automatically owned by the root user. You are the root (admin in Windows parlance) user. It is expected by Syncthing that it should all be owned by the synthing user. Manage everything for sync points/folders by the Syncthing GUI which runs as the syncthing user.

Everything Syncthing handles for sync’d files, folders should be managed by the Syncthing GUI & that data would be housed/stored at /mnt/mmcblk0/syncthing .

Beyond that there’s nothing more I can do here.