Not sure if I can help with this. But maybe I can give some insight to something.
Could it be that your board just got an AHCI limitation? In this case, I'd suggest you not using AHCI and maybe NVMe. Now I am not sure if your board supports NVMe. Some people even mod their UEFI to get NVMe support. Now since one drive will use NVMe in this case, and the other six drives will use AHCI, you might not get into this problem, and you will be able to have 7 drives. The only down side is, you would have to get NVMe m.2 drive.
Another solution could be that you get a mass storage PCIe card. Well a RAID card that works with MacOS. But you might not need RAID. In this case, you don't use the RAID function the PCIe RAID card gives. You can get inexpensive RAID cards, such as HighPoint Rocket Raid RR2720SGL. You can hook up SATA drives with it with SATA connectors. If you only got m.2 to connect, it must be SATA bu with an enclosure having SATA connector (not sure if those enclosures exists). There is a downside to this as well, the RAID card will show its own bios signs before booting, just like the motherboard does, this will of course make the booting experience a bit downgraded taking a few seconds more. If this isn't a problem, then this might be something you might want to consider. The RR2720SGL is quite inexpensive, last I checked it was about 160 USD. It got two SAS connectors, and can run 8 drives. All you need with it is mini SAS to 4x SATA 3 cable, either one or two of them, depending on how many you want to connect. Assuming you won't connect many, if this solution is for you.
The RR2720SGL works out of the box for Mac OS X / MacOS. However, I have only used it to macOS Sierra 10.12. I don't use it anymore, and therefore don't know if this will work for High Sierra, but I am assuming it will.