Have you tried installing the respective agents inside the VMs that assists with the guest interfacing?

• In VMware: VMware Tools
• In Hyper-V: Linux Integration Services (LIS) or hyperv-daemons
• In VirtualBox: Guest Additions

These generally help with the mouse input lag and host-guest interfacing that can be sometimes slow in virtual consoles, etc. I don’t know to what degree you consider ‘perfectly smooth’, so you may be talking about something beyond that but I hope it helps.

I did an in-place upgrade of a three-node cluster from 7.4 to 8. Followed the upgrade docs and it worked without a hitch.

I haven’t but would be curious to follow your journey. I’m running a NixOS VM (in Proxmox too) but still learning the basics. My goal initially was a simple utility VM. Separately I’ve also been moving my Docker host VMs to increasingly better setups and NixOS seems like an interesting next candidate.