> Unless you plan to power down most of the drives most of the time

I do. Read my other comments.

Yeah, it looks like you have somewhat niche requirements and would need to build your own hardware and software.

To run 10+ sata drives you'll either need to take a lot of care they're not spinning up at the same time, or getting used in parallel, or you'll need to dimension your power supply to cope. A beefy power supply will have a higher idle draw making the focus on getting the whole system idle down to 5w pretty hard