SilverStone has introduced the ECS07, an unusual device that resembles a thick SSD. However, the SilverStone ECS07 brings expands SATA 3.0 connectivity within a small form factor. The SilverStone ECS07 supports five SATA 3.0 ports running at 6 Gbps each; pricing has not been announced yet.https://www.notebookcheck.net/SilverStone-ECS07-transforms-an-M-2-2280-slot-into-a-5-port-SATA-3-0-expansion-card.634820.0.html
This is really cool and had it been available a few years ago I may have gone this route instead of the SAS-SATA HBA card I ended up going with. Unfortunately, they chose to use JMicron, which is generally agreed to be the worst of the three controller options. They should have used Realtek or at least ASMedia.
"Presumably, the device will not come cheap"
There are similar devices on Aliexpress, using the same JMB585 chip and some equipped with a heatsink, which sell for around 30€, so I don't really expect this one to be especially expensive.