um... anyone? 😞
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.
