Random IOPS through a SATA interface is orders of magnitude faster on NAND vs a spinning disk. A SATA SSD and a spinning disk might as well be two completely different categories of device for any non-sequential workload.
You can do it with PCIE switches, but you shouldn't. If you need 16 NVME drives, you need to put them in an appropriate host system.
To properly design a computing solution, first you define the requirements, and then you select components with specifications that will fulfill your requirements.
If you try to work backwards, you are destined for failure. Dropping big cash on 16 high-capacity SSDs just to ham-jam them in an old system is a really really dumb idea, especially if you're concerned about IOPS.
Are you not aware that the region between "saturating 64 PCIe channels" and "spinning disks" contains a huge space of performance, much of which might be acceptable?