NixOS in LXC works great, although I switched to bare metal NixOS a few months ago. I didn’t see the need for proxmox as it hindered my ability of declaring the whole system.

Creating NixOS LXC’s is a bit of a pita. Some links that helped me two years ago:

• wiki.nixos.org/wiki/Proxmox_Virtual_Environment#L…
• web.archive.org/web/…/nixos-proxmox-lxc/