Account is already linked SOLUTION
Hi all, came here to try to find a solution to my Xbox Account Already Linked issue and found nothing, and after a LOT of tinkering, I've found a solution, and here are the steps:
Note: you'll need a second email address
Unlink any other accounts you linked to your account
Change the email of your account to your 2nd one
Change the username to something else (I added _dummy to mine)
Click on Delete Account, follow the steps, and go to the homepage
Sign in using the account you want to link (In my case, Xbox)
When prompted, select "create a new account" using the old credentials (email and password)
You should now have a new account linked to your Xbox account, with your old credentials. You can now link your other accounts like Epic and Steam for the rewards
Personally, when I launched the game after that it signed me in automatically, with my account
Let me know if this helped you!