[Wuzado]

While YMMV, some smaller companies and manufacturers offer boards based on ARM or RISC-V cores. Consider checking out the MNT Reform - likely one of the closest computers that while a bit impractical, fills the openness criteria.

[Rinzler89]

Modern ARM designs destined for computing are alo coming with management engine like features, just like Intel and AMD, plus other obscure silicone features and proprietary FW blobs, even more so than Intel and AMD, like DSPs, ISPs, etc.

If you want non-obscure HW you'll have to go back to pre-microcode days of processors roll out your own.