This sounds like a lenovo machine. Or something with a similar MOK enrollment process.

I forget the exact process, but I recall needing to reset the secureboot keys in "install mode" or something, then it would allow me to perform the MOK enrollment. If secureboot is greyed out in the BIOS it is never linux's fault. That's a manufacturer issue.

Apparently, some models of Lenovo don't even enable MOK enrolment and lock it down entirely. Meaning that you'd need to sign with Microsofts keys, not your own. The only way to do this is to be a high-up microsoft employee OR use a pre-provided SHIM from the distribution.

https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot#Using_a_signed_boot_loader

For that case, Ubuntu and Fedora are better because, per the Ubuntu documentation they do this by default.

On Ubuntu, all pre-built binaries intended to be loaded as part of the boot process, with the exception of the initrd image, are signed by Canonical's UEFI certificate, which itself is implicitly trusted by being embedded in the shim loader, itself signed by Microsoft.

Once you have secureboot working on Ubuntu or Fedora, you could likely follow these steps to enable TPM+PIN - https://wiki.archlinux.org/title/Systemd-cryptenroll#Trusted_Platform_Module

There might be some differences as far as kernel module loading and ensuring you're using the right tooling for your distro, but most importantly, the bones of the process are the same.

OH! And if you aren't getting the secureboot option in the installer UI, that could be due to booting the install media in "legacy" or "MBR" mode. Gotta ensure it's in UEFI mode.

EDIT: One more important bit, you'll need to be using the latest nvidia drivers with the nvidia-open modules. Otherwise you'll need to additionally sign your driver blobs and taint your kernel. Nvidia-Open is finally "default" as of the latest driver, but this might differ on a per-distro basis.

I was going off what you said:

my threat model involves someone trying to physically unlock my device

This doesn't sound to me as if you're concerned about espionage - repeated, covert, root access to your computer, for the purpose of installing software to capture your keys, so that they can steal your computer and have complete access. If someone has remote root access to your computer, you're fucked, TPM or not; they'll just read what they want whenever you're logged in and using your computer.

TPM is for when you might not have secured physical access to your computer. Like, you're worried the NSA is going to sneak into your house while you're out shopping, pull your HD, replace the boot loader, and re-install it before you get home.

If you're only worried about, say, losing a laptop, or a search & seizure at your house, an encrypted HD is good enough. TPM and a keylocked BIOS are belts-and-suspenders, but if they want to get at the data they'll just pull the HD and run code-breaking software on it on and entirely different super-computer. TPM won't help you at all in that case.

Honestly, TPM is for a specific threat mode, which is much more like ongoing espionage, than simple opportunity theft. Your stated use case sounds more like the latter than the former.