Can't boot from Big Sur installed on a USB
Get a new drive. That process is quite a bit longer than you might think you need, but it’s the safest and most direct correct solution. If you have errors with the install it’s likely these are the causes in order:
- Bad USB storage
- Bad download of the installer (very unlikely, but it happens)
- Not following the guide
- Failure of the Mac (more likely USB port or cable or hub)
Catalina and Bog Sur are well vetted now. When you asked this, Big Sur is still getting some edge cases patched up, but either should work well now for anyone facing this situation.