Did you read my first post? I absolutely can use GDrive via Dolphin/kio-gdrive. Read the HOWTO I linked how I did it, and how to avoid the issues in the two posts you linked. It’s an individual solution which also got mentioned here yesterday: KDE Online Accounts - Not Signing In - #89 by johnandmegh
Messy because the access token is frequently forgotten and I need to re-do the setup, but it works.
My issue is was that it works only on Kubuntu, not on EndeavourOS (which I use more often).
Just during writing this post, I accessed the account via Dolphin to verify that it still works.
EDIT: And to spite you and me, I retried on EndeavourOS, and the signon process and subsequent access of GDrive went flawlessly. I add my edit to the original post so I can mark it as the “solution”.