I had totally forgotten about #syncthing, so with thanks to @ryen and @email@example.com for reminding me, and for linuxserver.io for having their ready-to-roll images, I just spent under ten minutes and now have my photos directory syncing from the synology to both HC2s.
Tomorrow = set up the remainder of my "need to sync" directories, wait for "some time", and then take the designated "off-site" HC2 to $secretDestination
A bunch of technomancers in the fediverse. Keep it fairly clean please. This arcology is for all who wash up upon it's digital shore.