Yeah, I wouldn't recommend a raspberry pi for anything beyond trialing it out (unless you go with a ssd hooked up as the logging kills the cards quick in my experience). If you've already got a mini pc with a half decent CPU and enough RAM, I would give proxmox a go and use the community scripts. They give you all the stuff you need and the defaults OoTB for proxmox will probably be fine for networking, storage, etc. I think home assistant gets set up as a full VM and plex is an LXC but if you just paste in the command, it'll talk you through the options (usual thing of being careful what you paste into a terminal, especially with just running scripts from github but community scripts has been fairly well curated).
Only thing that might be complicated is probably plex if you need to hook it up with storage via nfs or something as that can have headaches in lower permission containers. So long as you have free reign to have a bit of an experiment on that mini PC without losing anything, should be up and running fairly quickly I'd say.