Did you try OpenBSD with bioctl?
You can tamper with the bootloader, but not the rest. And you can always set the bootloader in another media and always boot from that.
Tampering with the bootloader is game over. And what, are you keeping this other bootloader medium on your person and in your sight at all times? It's never ever unattended?
Using TPM with closed source firmware, especially written and designed by Microsoft, probably full of backdoors, when you don't even know what it's doing is a worse choice.
I don't think it's a worse choice. Either way, you're screwed with physical access. At least with TPM, the attack requires more sophistication.