in my case i don't see any performance decrease - i'm running unraid as pure nas + some dockers, including Nextcloud, Plex, duplicati..
i have no experience with AMD at all, but should work just fine. for VMs i prefer SSD, and in my newest server PCIe NVME drives, and i don't use any RAID, insteed i do regular backups for my other than unrad VMs to unraid over NFS.
No, i have 10Gbit nics and 1Gbit nics in my servers and i'm using them in Active-backup bond mode on host side. all my VMs have virtual nics only and thats working just fine.
if you go Proxmox or ESXi, you can assign more cores to VMs than you have total Cores available on server. just watch for usage - i think, your will never run at full speed all of your VMs at once. i have a 8 Core CPU with HT enabled, so host OS sees them as 16 CPUs.
you don't need a GPU at all to run Plex, just avoid any transcoding. i'm running Plex as Docker on unraid VM and it runs just fine - i assigned max 6 of 8 CPU to Plex docker to not overload unraid VM itself, if in same case plex would start some transcoding. but i don't have any 4K content, 1080p/720p only, so i can do some transcodes to lower settings in CPU without problem.