At this point, on this platform, I think your fastest bet to mostly open sourcing it all is linuxboot. We recently had an experience where we installed a linux kernel in FLASH on two new boards in two days and most of that was just figuring out how to rearrange the UEFI bits, (i.e. move the furniture around :-) not building code. You can now replace a lot of UEFI with a linux kernel and the only thing you have to build is ... a Linux kernel.
We recently found that for supported boards, a git clone of the linuxboot repo and full build takes 2 minutes 45 seconds, and that's essentially hands off.
If you have a UEFI system, which that board almost certainly is, I think you can skip coreboot and u-boot entirely and just take the linuxboot approach. I'm no longer that big a fan of FSP, it has its own problems.
I realize in this note there's a lot of "Alphabet soup" (many, many names like UEFI and FSP and all ...) but the short form is this: with modern x86 CPUs, the coreboot port is indeed a very large effort. The linuxboot effort is, as mentioned, as little as a day in some cases. I can tell you from experience it is far less work and, ironically, can also result in the use of fewer binary blobs on these CPUs.
Obviously, for open CPUs, I still prefer coreboot; but x86 CPUs are no longer open in any meaningful sense of the word.