Your idea for using TPM as host verification makes a lot of sense to me - as it stands right now, I'm playing around with this for a side project right now, and a TPM-backed key which is bound to the right PCRs should give you assurance secure boot is enabled, with your own custom keys, and that it loaded your signed Grub, booting your signed kernel.
I also played around with tpm2-pkcs11 last year and it worked nicely, and has support on a lot of devices like the XPS (which works really well on Linux!) Indeed, PGP smartcards should work too, as should cards using the open-source ISOApplet.
