Wait for systemd oneshot service to finish

Found the answer here: https://unix.stackexchange.com/questions/216045/systemd-configure-unit-file-so-that-login-screen-is-not-shown-until-service-exi

Just set Before to when the terminal is available:

[Unit]
Description=Prepare the system after installation
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[Service]
Type=oneshot
ExecStart=/usr/bin/prepare-system

[Install]
WantedBy=multi-user.target