Look like you did t change your discord name when was this discord name migration? Try to change your discord name and disable that "Legacy Username" from your discord settings.
Settings - Authorised Apps -Electronic Arts
If you have it authorise then deauthorise and try again.
my discord username is their current format. I dont know why its adding the additional #0, since I dont have a discrim tag.
I did the reauthorization process you said and tried to link my account, but received the same error again.
Maybe that post helps?
https://forums.ea.com/discussions/ea-app-technical-issues-en/re-to-continue-linking-your-ea-account-head-back-and-start-over-/7623075
