I don’t know about proxmox but I have 6 Pi4 8GB, 5 lenovo Tiny (i5-4x and i7-8x), and 3 laptops in a k8s cluster and love it. I actually have too many hosts but it’s a lot of fun for me to play on them. Storage is longhorn for anything with a database, everything else is over NFS onto a box I built myself. If you use the *arrs try them over the network and watch the logs. They HATE network storage and will complain a lot. If you don’t have any issues there you should be good. I haven’t had any issues and just use the 1gb network. As for form factor, I think it’s whatever you have room for. The laptops are annoying but they have the best hardware so I put up with it.