Run the following commands on your target system:
wget "https://apt.luthienstar.net/amd64/luthienstar-repoconfig_1.0.0_amd64.deb" sudo dpkg -i luthienstar-repoconfig_1.0.0_amd64.deb sudo apt update
That package will provide the necessary configuration to set up the repository and its keys.
On top of it, you won't need to manage the repo signing key update.