AFAIK there's nothing preventing X or X in VNC from running as a system (root) service. That's how a lot of graphical containers are structured. I wouldn't do it on baremetal (OP's question is an X-Y), but I don't think anything explicitly prohibits it.
I was under the (maybe erroneous?) impression that running X as root is no longer permitted on modern X11 versions, but I haven't actually tried it since rootless X became an option. If you're right that it's still possible to run X as root, then that's another potential solution (although there are security risks, obviously). Thanks for the tip!