I don't know about low power (or inexpensive, for that matter), but the HiFive Unmatched has lots of IO. 8 lanes of PCIe 3.0 can be expanded into quite a few SA

I don't know about low power (or inexpensive, for that matter), but the HiFive Unmatched has lots of IO. 8 lanes of PCIe 3.0 can be expanded into quite a few SATA 3 ports. For ARM, rk3568 is available now and has 2 lanes of PCIe 3.0 and up to 3 SATA-3 ports. As opposed to the rpi4 which has a single PCIe 2.1 lane.