Those are all valid manual ways but I want to have my node do do this automatically. I should have been more precise in communicating my final goal:
I want to set-up the node in such a way that - once it has a power failure and power comes back - everything auto-starts without me setting groups, links or permissions.
This is possible and i have it set up that way. Theres actually an easy guide, that worked flawlessy for me, you might wanna look into it. You dont need a script btw.
https://raspibolt.org/guide/bitcoin/bitcoin-client.html#create-the-bitcoin-user