managed to get some help in the gameforum (thanks again, scuba! 🙌 ).
so, for all you other poor fools trying to do what i wanted to do (and confused by old threads and conflicting info): it is not possible. you need to create a new google-account and link the guest account with that. here's what you need to do:
- make a new google account (you can also use an existing one, but make sure that the game has not been linked with it before)
- start the game on your guest account
- go to settings (top left)
- click "connect"
- "sign in to google play" - you will get a warning screen, that's ok
- the game will start again and ask you if you want to link an account or play as a guest - choose to link an account
- select your new account
- it will ask you if you want to keep your progress - you obviously do
tadaa! you've made it. your former guest account is now linked.
