Yesterday I took the next step, perhaps it will serve you too.
I have a Gmail-account and I always had it in the Mail-app as one of my 4 mail-accounts. When I checked my mailfolder-list, I saw the mailfolder "Important".. It turned out to be one of the Gmail mailfolders. Huh? Google is deciding for me what is important?? I dragged the content of the Gmail inbox-folder to one of my other Inboxes of my other mailaccounts in the Mail-app. Then I logged into Gmail on the web, opened Settings en Redirect / POP-Imap. There I redirected Gmail to one of my other mail-accounts.
After I did that I went on my Mac to System Preferences > InternetAccounts > Gmail > unchecked mail. That was the last one. I checked if re-directing worked, it did!
Then I deleted the complete Gmail-account on my Mac in System preferences > Internet-accounts. Result: in the Mail-app the list of mail-folders was shorter and the "Important" folder from Google was gone.
Now I am able again to decide for myself what is important to me!😉