So, it seems a NAS with HDDs will never achieve the kind of throughput I need.
I haven't said that at all

. Although my guess is that the number of spindles does not deliver the required number of IOPS. You can achieve a pretty high number of IOPS with spinning disks, but you need very many of them. This is how data center storage used to be many years ago. If you roughly get 300(?) IOPS from a single disk, and by that also from a mirror, you could certainly achieve more than 100k IOPS. But will need probably more than 7k disks (if I calculated correct in my head).
What I meant to convey was that you try to quantify what your use-case means in terms of required performance. Such a sizing is what you typically do (not only in the storage context), when it is not exactly clear what performance is required. Think like "someone scrubbing the timeline in a video needs a certain response time (typically milliseconds) from the storage system until the desired position in the file has been reached; from that point on a sequential read will occur that requires a bandwidth of 300 Mbit/s". Whether this is exactly what you need (esp. the 300 Mbit/s) I don't know (it's been decades that I did storage professionally). But hopefully I could bring the general idea across.