The issue is that Google does most of the work creating Coreboot ports for recent Intel chips, because Google uses Coreboot in its Chromebooks, and when Google asks Intel for detailed documentation and some programmers to help port the firmware, Intel responds, because millions of Chromebooks are sold every year. Unfortunately, Google only cares about porting Coreboot to the mobile versions of the Core processors because they are used in Chromebooks. If a tiny company like Purism asks Intel for help porting Coreboot to its desktop Core processors, Intel will probably ignore the request.
If you want to use a recent Core processor with Coreboot, you have to use a mobile chip (which is soldered), and most mini-ITX motherboards use desktop processors. However, I did find one system that looks like you want: Fanless mini-ITX with mobile Core processor, but the most that the mobile Core processors support is up to 64GB RAM (except for the i7-10750H which supports up to 128GB RAM).