With Highend boards than can split into Gen5 x8/x8 xou wouldn't even have to ditch the GPU. But it would be better to use an Gen3x8 SATA/SAS-Controller in a physical x16-Slot that is atleast x4 electrical so you have ~3.5GB/s of bandwith, which is theoretically enough for 6xSATA-SSD and will be plenty for double that number in HDD. There even are Gen4x8-Controller but they cost a lot of money.Only other option is ditching the GPU and buying an x16 mini SAS HD HBA card
I have seen those with up to 16 ports and with 16000 MB/s you dont have a bottleneck even with all 16 ports in use
There doesn't seem to be any demand for high-bandwith SATA-Controller outside of very potent SAS-Controller. Marvell stopped developing their SATA-Controller around 2013, so they are still stuck on Gen2, while Gen3x2 is enough for 3 SATA-SSD or 6 HDD (even the most advanced HDD don't reach 300MB/s sustained).