Adding to this: doesn’t CAD usually want 3D acceleration? I would definitely try running the CAD software with the same VM configuration you plan to use in your Proxmox VPS first before progressing to make sure it (a works at all and b) is responsive enough. You could even try nesting Proxmox in Proxmox to emulate the kind of performance you’d had on a VPS.
Last time they’ll ever do that! Pass the buck of hosting web-facing Plex servers onto somebody else.