As an update I seem to be having success with the repository linked further down in the GitHb comments:
which seems to be a much more thorough attempt to support Raspberry Pi well, though I barely understand any of what it’s doing.
The SD card image generation worked great, then adding a package to the right place (environment.systemPackages) and running the suggested deploy command installed it:
nixos-rebuild switch --flake .#rpi5-installer --target-host root@host
The new configuration did survive a reboot. I suppose that the key is using the same flake for initial installation and later deployment, since somewhere in there is a part that updates the boot manager to boot into the current configuration. Like I said, I don’t understand much of it.