Thanks for that absolutely moronic and completely useless comment.
For any sane person that might stumble upon this thread having the same problem as I do:
I needed to set the OpenVPNService to start automatically as well as create new scheduled task ( refer to: https://strongvpn.com/autoconnect-windows-10-openvpn/ ). In contrast to the aforementioned guide I set the task to start "When the computer starts" and made it "Run whether user is logged on or not".
Running the service as well as the scheduled task in conjunction let's OpenVPN correctly connect on startup before any login. Running the service or the task on it's own both isn't working. While the service alone won't do anything, the scheduled task running on it's own connects to the OpenVPN server, but seems to have problems with some parts of the config since I'm then not able to access the internet.