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.
The only part that needs to be repeated is adding the account in Online Accounts. The only part of that process that Google controls is the OAuth login/permission flow. You’re technically correct that it’s “subject to change”, but Google has always been very conservative about changing those pages: they believe that not having their auth screens change is important to helping people avoid phishing attacks.
(That said, my guess is that going through the OAuth flow will be hard to automate. Someone should really implement token refresh…)