You should be able to fix your local user name in lusrmgr.msc (select your account and press F2). I’m using a Microsoft account, but to avoid this nonsense, I

You should be able to fix your local user name in lusrmgr.msc (select your account and press F2). I’m using a Microsoft account, but to avoid this nonsense, I created it as a local account (because this laptop shipped with Windows 11 Pro RTM, so I could still do that) and converted it to a Microsoft account. If I didn’t have this option, I would probably create a Microsoft account with the dumb default name, then create a local account, delete the old account, and convert my new local account.