Most of the boxes sold here support 2.5G. No mention of Coreboot, unfortunately,
but could be worth asking if they can install it as they already offer OpnSense or other firewall software installation as a service.
Installing it isn't really enough, you want updates. Protectli provide instructions to compile it yourself, so even if they stopped providing binary releases, you could DIY it; well worth the premium for the security conscious in my opinion.
I think they should upgrade their NICs to 2.5G and 10G though. 1G is no longer future proof.