The latter works, thanks. But, Frontier still sends a verification code to my ex's eail, which is not ideal. I'll have to explore some way of getting the game linked to my own email (but might have a problem there are I already have the three other accounts linked to various email addresses and presumably Frontier won't allow two accounts to share the same email address).I'd imagine that if nothing else you could buy the game on you own steam account, and then link and log in with the old account's email and password?
If it's an older account (don't remember the date it changed, but 2018-2019), chances are that you could log into it with Frontier's launcher too.