I worked around this by making a script, that creates a container from template tarball, runs nixinate on it with minimal config, populating the store and profiles, reboots the container, runs nixinate again to finalize configuration switch and converts it to Proxmox container template. It’s not the cleanest solution, but it’s simpler than I expected and it actually works how I want. As for making a full tarball from scratch, the search continues, but maybe at another time.