So the workaround is running the SFTP process as root?
Why not run the SFTP server as a docker container as well (e.g. with https://hub.docker.com/r/atmoz/sftp/ )? You can mount the same volume in the SFTP container, and have it listen on some random port. Just make sure to configure the SFTP container to use the same uid:gid as the one used in the syncthing container to avoid file permission issues.
In that case, perhaps replacing
-o sftp_server="/usr/bin/sudo /usr/lib/openssh/sftp-server"
with-o sftp_server="/usr/bin/sudo -u <syncthing_user> /usr/lib/openssh/sftp-server"
is a good compromise?