I don't see a reason to segregate the two things for my setup, so I'm going with start9 & seeing if it can handle everything I need. But I can understand wanting something dedicated.

Because of the mini PC physical layout, I can't directly connect the mirrored drives via sata. So they'll just be usb3. So: slow. And not going to buy a dedicated NAS box. So I don't want to treat those drives as my main data drives. They'll only be for backups.