Won't be close to working. I can say that without even looking at what you are suggesting. It is not going to happen.
Any effort do make a Thunderbolt upgrade in a Mac Pro Tower would require a special GPU board with integrated Thunderbolt. Which means it is way too much work to justify anybody doing that for a small and dying market. You can thank Apple for jamming graphics into the Thunderbolt spec to make it effectively impossible to backwards retrofit. And in this particular case, how exactly did you think this will work? A USB 3.1/USB-C PCIe card in the Mac Pro? That gets you USB 3.1 on a USB-C connector. Which is uh USB 3.1 not Thunderbolt anything, even though Thunderbolt 3 can use the same physical USB-C connector.
And you want to connect that to a Thunderbolt 3 on USB-C to Thunderbolt 2 (or 1) on a Mini-Displayport connector? Well that will do exactly nothing, nada, since you are not running Thunderbolt 3 on the USB-C connector, you have USB 3.1 on it. And in case you are thinking of other ideas, you cannot go from USB 1, 2, or 3.x to Thunderbolt, you *can* go from Thunderbolt to USB. Download whatsapp for mac os x 10.4.11.
Installing a Metal compatible card into a Mac Pro 5,1. Take the two Mini 6 Pin to 6 Pin PCI Express Video Card Power Adapter Cables and plug them into the two power ports on the mother board. You should already have one cable from the old card which you can use or replace. Upgrading your Mac Pro 5,1 to USB-C. December 19, 2018.