I don’t know much about OAuth, but after I did everything described in the post, I still got an error:
The problem was the fact that I didn’t change the Client secret in <setting name="ClientSecret"> field of the local google.provider document to the new one. You can get this one in the same place you got Client ID. After you insert the new value and refresh KDE Settings app, everything works as intended.
Maybe that helps some people…
As for me, everything described in the OG post works as of March, 3 2026.