P.S. Apparently it should work, but you have to flash the Gigabyte GC TitanRidge PCIe card: https://youtu.be/R6jrc3TxQD... Certainly not easy...

Absolutely not. The card requires a Thunderbolt header on the mainboard, which only very few have, but Mac Pro 5.1 certainly does not. With Mac Pro 5.1, it is only possible with the "Gigabyte GC TitanRidge PCIe card", but this is very problematic in terms of software: https://www.logicuser.de/forum.... Perhaps there is a solution here: https://create.pro/the-mac-pr...