environment.systemPackages = [
(pkgs.writeScriptBin "nixos-install" ''
  export SYSTEMD_RELAX_ESP_CHECKS=1
  exec {pkgs.nixos-install}/bin/nixos-install "$@"
'')
]

https://github.com/systemd/systemd/issues/12468#issuecomment-698782077

And yes, sd-boot writes to the ESP if boot counting/boot assessment is enabled.