You probably want to run all of that on bare metal in containers with Docker or Kubernetes.
Containers let you easily share resources between them, because they all share the same kernel. VMs are harder to share hardware resources with, as you’re finding out.
I was not sure if I should run a LXC container for each docker, or have a single LXC with everything (exception xrdp / XFCE). I don’t know what would be good practices…
LXC is a container. I don’t think you would want to run Docker inside LXC. That’s running a container inside a container. I’m a noob though.
Normally, you run one app per container, or one set of apps per container if they are closely related. You could run all the Plex suite apps inside a single LXC container and Windows alongside it in Proxmox. Or you could run each app inside their own LXC container.
Alternatively, you could run them all in individual Docker containers on bare metal Ubuntu, but not have the ability to install Windows or other OSes.
The most important part of a Frigate NVR is the Coral Edge TPU. As long as you can secure one, you’ll be set. The USB version is easiest to use, since it doesn’t require installing drivers and most PCs will have a USB 3 port.
All the processors are good enough for a small NAS and NVR.
I wouldn’t buy a PC without SATA ports or an expansion slot. If you go the mini-pc route, where would your store your HDDs? An expansion slot would let you one day add an HBA card for more SATA connections. By default, it looks like all those motherboards have limited SATA connections. The 7080 Tower says it has 4 SATA connections, but only 1 of them looks like its the full speed SATA3.