Right, but if you are on Pro Tools HD 10 you can't upgrade beyond what? Mountain Lion/10.8 and so can't get NVMe support in macOS, which you'll probably want to be on Mojave or later. I'm not even going to look up their exact details because you will be so far off. And NVMe options won't work for you. (AHCI based M.2 drives would work but have long since been replaced by NVMe and are effectively unobtainable). Of course if you were going to give up HD/TDM then by all means I'd move to NVMe SSDs, they are fantastically fast.
So I'd assuming the best options for you are to start looking at all the 2.5" SATA drive mount options and different SATA III cards.... and for that any of your slots will work (although if you want the SSD on cards you need more physical slot space... and may not say want to have the drive up against a hot running GPU).
Like
https://www.sonnettech.com/product/tempossd.html or
https://eshop.macsales.com/shop/SSD/...sior/S-Carrier
But my personal bias would be to not mount the SATA drives on the PCIe card, largely because I'd want multiple drives. In which case just about any SATA III card will work, and using mounting options from folks like OWC:
https://eshop.macsales.com/owcpages/multimount