I've tried to post this in Development/Package Issues, but the button is "Can't Start Discussion" instead of "Ask a Question".
After installing Keysmith successfully and starting the programme there was the following error:
keysmith[81348]: QQmlApplicationEngine failed to load component
keysmith[81348]: qrc:/main.qml:8:1: module "org.kde.kirigami" is not installed
Installing packages kirigami2, kf6-kirigami
would still deliver errors.
Only after also installing kirigami-addons
it worked as expected.