What is most modern laptop with AMD cpu (AMD PSP doesnt have access to the network, Intel ME has it) that supports coreboot and qubes? I dont think buying already corebooted laptops from privacy focused vendors is a good idea, since customers are probably monitored by feds
DId you have a look at the HCL?
You could also buy pre-owned devices of this kind.
Without the source code, I would not be so sure.