So with a Mac Pro 5,1 you have pretty good PCIe/NVMe SSD upgrades options as long as you have a decent spec PCIe slot spare (e.g. 8 lane) and you are able/willing to update macOS etc. to support NVMe. And that macOS and Pro Tools updates may mean this will *not* work for you since I'm gathering you are on PT 10 and HD/TDM not HDX.
--- but if you do want to go ahead....
So what exact physical and electrical specs are the free PCIe slots you have? (or what are the best you can get by rearranging stuff?)
Just about any Samsung NVMe drive is a great choice. I'm running 980 Pro in Thunderbolt 3 expansion chassis nowadays, maybe overkill for the 5,1 ... but check prices, I might go there if they are not much more than others (I'm using them to standardize across M.2 drives on other systems... connected via PCIe 4 on a AMD Threadripper).
You will need an M.2 to PCIe adapter card and you ideally want one that supports switching that does PCIe 2 to 3 link aggregation. Like a Syba card... but what exact spec slots do you have available?
There is a lot of information online about updating Mac PROMS, macOS etc. to support NVMe. Start here:
http://blog.greggant.com/posts/2018/...ade-guide.html
--- otherwise you are going to have to go with a 2.5" SATA based SSD, like a Samsung 860 Evo, and ideally find a PCIe to SATA III adapter card. Those cards either directly mount the 2.5" SATA drive(s) on board or you can can find optical bay adapters to mount the drives there or with more work find ways to mount them in the old internal drive bays... which are only SATA II, you can connect a SSD there, but it's painfully slow. Sonnet and OWC sell different cards and mounting options, but I'd use a Samsung drive over OWC's own SSDs.