Hmm, before taking the nuclear option I would verify if the problem is indeed the W11 mail app or a BT problem.
Even if you don't wish to use Thunderbird going forward I would see if it fixes the problem or whether the problem still exists.
I have just tried Thunderbird and was able to use it to access my BT email so I can access my BT email using Outlook and Thunderbird but not the Windows 11 Mail app. I'm convinced the fault is with the Mail app.
Since I can use Outlook I am not going to reinstall Windows. I'm not that attached to the Windows 11 Mail app.
Sounds like the sensible thing to do rather than taking a sledge hammer to crack a nut.