Had this EXACT problem trying to fix our old Microsoft business account at my last job. Last guy ran off with the creds, wouldn't answer his phone for 2FA.

Poked at it on and off for a couple of years, finally spent a full 8-hour day banging on it. Fail. Boss: "Yeah, didn't think that would work."

Y'all I am tenacious when you tell me I can't do an IT thing, won't give up. Microsoft left me a broken man.