I have a QNAP NAS that I put a bunch of SSDs into to make it insanely fast, but that also means it doesn't have a lot of capacity. I'm not sure how easy/difficult it is to swap the SSDs out for large capacity HDDs.
I wish the Drobo I bought a million years ago actually worked like it's supposed to instead of randomly becoming unresponsive. That thing made it dead simple to swap drives. Literally just yank one out, slap a new one in, and wait.