I managed to get TorizonCore to boot from an SSD by telling the initramfs to look for the TorizonCore partition on my SSD using the tdxargs U-Boot parameter.
It actually wasn’t that hard once I knew what I was doing, and I’ve gotten great performance so far.
I built Torizon in Yocto (standard build, absolutely nothing special) then edited the image.json file from the torizon-core-docker-blah-blah-Tezi-blah-blah.tar to only include the bootfs for the eMMC. I then used the same Yocto Tezi build to extract both the bootfs and rootfs to the SSD (you must be root when extracting).