> PPC64le Talos II workstation which has a fully open hardware motherboard and open CPU architecture?

The ISA is open source, not the whole CPU architecture and design. There are older open core designs from IBM but that's a different thing from the more modern and powerful Power9 and Power10 CPUs.

> you still only have to trust your CPU vendor even there, as it is possible to have FOSS firmware/software for everything else

A device with assorted closed source components including as part of the motherboard itself is hardly open beyond the CPU. Open source also doesn't mean you aren't trusting those vendors. With a fully open hardware design CPU, you're still trusting that it matches the open source design and you're trusting the open source design. The manufacturing process is also generally going to be proprietary.