If this is your stance it's trivial to have your cake and eat it. You can easily build your own GrapheneOS based image, which will then be unable to use attestation (because your own keys you'll use for the secure boot won't match).

And I'm not trying to be snarky, if that's the only thing, it's solvable right now. Everything else will work.

And concerning "you can't just buy a device and install is" - android and iOS are far more secure than any desktop os, and both pixel/iPhones are far more secure than any computer (or any other phone as well (we'll get the third one next year)). GrapheneOS actually explains "whys" in their hardware support faw section.

Generally maintaining an os is a hard work, so that perhaps explains why there's not many mature offerings.