But.... Citrix.
In all seriousness, I'd have a look at Citrix now that Cisco has their grubby hands off of it. Could be contender if they add Proxmox support.
I tried this recently. It has a ton of promise, but felt very hacked together. We had a lot of stability issues during our 60-day trial. I might have another go with it in a year or so, but I didn't want a system I had to babysit all the time.