It can be, but one of the biggest draws to things like Sandstorm and Yunohost is that they handle auth for you. I run a bit of Dockerized infra too but managing users for a bunch of services separately is kind of a pain. You can set up central auth yourself but as a lone sysadmin that's even more of a pain :)

Fair. Could an auth solution for proxmox not exist? Or Sandstorm/Yunohost run within proxmox?

I remember seeing a Github repo where someone had baked all the proxmox install and maintenance scripts using rockylinux including auth. Much cleaner than I would organically end up at without reinstalling :)