A Desktop suggestion with various independent USB controllers, an open source firmware, at least 32gb RAM and possibly a nerfable microprocessor to disable ME or equivalent.
No gaming, no hyper speed. Just working well with Qubes.
Well it seems basic requirements, but I doubt it is easy.
Three years ago Taiidan suggested an ASUS KCMA-D8, keeping in mind the above requirements. See this thread: https://groups.google.com/g/qubes-users/c/MEMdWdsht5k
But three years passed and possibly even the above board or the processor changed, or something better had been released. Things change, even Taiidan disappeared from this email list.