Are you exposing any ports to the internet? The older library versions in High Sierra have numerous security issues (of various severity). You might be able to reduce your risk by disabling server features, but that's a dangerous game of whack-a-mole. (Also, as a fellow self-hosting fotog (that never went pro): you should try PhotoStructure! Details in my profile).
Yea, no kidding, I wish apple would open source server.app, so some of the old components could be updated without hacking it. It was just a perfect all in one solution. I will check out Photostructure.
