If you want to make reinstall a little easier, turn off FileVault before you do the reinstall. and then follow my original post instructions. Once you get Big Sur reinstalled, you can turn FileVault back on. Once you have finished the install, go to disk utility and rename the 'untitled' disk to Macintosh HD or whatever name you wish to use. That is the simplest way to go about it, in my opinion.That's cool just Filevault should be on every Mac, and this doesn't rename the disk correctly, it's all just a little messy.