I still don't understand why I simply can't get the GetGoogleIdOption to work but if I use the GetSignInWithGoogleOption instead, everything seems to be working as expected. All I needed to do was changing this:
val googleIdOption = GetGoogleIdOption.Builder()
.setFilterByAuthorizedAccounts(false)
.setServerClientId(getString(R.string.default_web_client_id))
.setNonce(hashedNonce)
.build()
...into this:
val googleIdOption = GetSignInWithGoogleOption.Builder(getString(R.string.default_web_client_id)).build()
I hope this answer can help other devs that are facing the same problem because (sorry Google) the documentation is NOT very clear... at all.