I think there are some misunderstandings in this thread about what Secure Boot is and how it works. Secure Boot doesn't protect your disk encryption key.

The purpose of Secure Boot is to validate that the bootloader is trusted so that you can have some assurance that you're not giving your disk encryption password to a fake bootloader which phishes you.

Secure Boot doesn't give any agency more control over your machine than if you were not running Secure Boot. Using Secure Boot is strictly more secure than not using it, even if you don't trust the parties who made the implementation.

You could say "Your computer is trustable, if you have verified the silicon of your CPU, motherboard, etc." and it would be equally true. Secure Boot isn't perfect, but it's a lot better than no Secure Boot.

The only argument against it is that it provides a false sense of security, which is only a problem if you decrease security in other areas as a result of using Secure Boot.