if you can, setup a stratum 1, but... regarding openntpd@freebsd; https://www.digitalocean.com/community/tutorials/how-to-install-and-configur...
The stratum isn't much relavant to tor relay nodes since they're tolerant by design of more offset than any sane sync would produce, and OP wanted to remain closed so won't be serving up time to others who might care.
More interesting would be putting in opposing pools and security protocols for discrimination and anti fuckery. But at that point you've probably got other threats beyond if you can sync consensus or not.
There is the bit about tor getting time from the tor network itself, don't know if that has been released yet. It would be randomized and not many layers, so likely also moot therein.