If you are writing a software stack from scratch, perhaps you should start with a relatively low-end RISC-V SBC. Once you have accomplished that, you would step-up to a more advanced SBC and implement the next higher levels of abstraction.

As you progress so will the RISC-V environment and choices. And the cost of SBCs will continue to come down. Buying the most powerful SBC now is an expensive overkill. When working at the low-level you won't be programming PCI and USB straightaway.

With your stated goals, your growing RISC-V expertise will become increasingly valuable.