Neither the RPI4 nor the SoC have a TPM, so to implement any FDE mechanism relying on TPM you would have to use a had. There are kernel fde hooks that can implement such mechanism, probably a matter of implementing those in your kernel snap. For starters in the snapd source tree, there’s a fde-setup-hook implementation we use in the tests. See UC20 full-disk-encryption hook interface for some docs on this.
As for UEFI boot on RPi4, is there some progress you’d like to share?