FYI I had a situation similar to yours, but luckily for me, the problem just went away and let me install Windows 11 via WU from my existing Insider DEV Windows 10 build. I have no idea why the problem just went away; I can only guess that there's an intermittent bug in Windows Update, but I have no proof. I hope that you will eventually be as fortunate as I was.
FWIW my PC is a Dell Optiplex desktop with an old fourth generation I7 CPU. It does not have TPM version 2 and it has legacy boot.