There's M.2 cards with PCIe to SATA bridges, which are great for a high-capacity low-speed NAS setups, especially with mechanical hard drives. Be sure to research the chipset before buying one, as some are notorious for poor driver support.

If this is a practical application, I would recommend sticking with ARM for this generation, as RISC-V is still in early development. In a year or two we should start getting more standardized RISC-V processor implementations, with broader support.

Ah, cool. I'd like the same, but with ECC RAM, so the options are even fewer.

Have you looked into what someone suggested here, splitting the NVMe to some SATA ports? It didn't sound like a horrible solution, but I have no experience.