I currently have 14 drives plugged into the backplane - 8 Seagate Exos, 4 Crucial SSD’s, and 2 Intel Enterprise SSD’s. However, I altered my setup a little bit. I swapped the cables around so I had 8 SAS connectors and 7 SATA.
So we have a few additional variables at play here (drive brand, SAS drives, SAS controller) in addition to the BIOS settings. Looking at my BIOS settings for the SATA ports, I think they are pretty much the same as the ones you shared.
Maybe try to move your drives to the slots 9-15 and see what happens using the SAS controller? I’ve had my HL15 for a few years now. If I find some time here, I can try to power it down and cold boot it to see if the SSU works. It’s always possible I’m misrembering something.