[samtrino]

As recommended above, I wouldn't purchase a Mac Pro 5.1 at this time just to run TB3 on it, but if you already have one and would like to extend its life for few more years than a (flashed) Titan Ridge works flawlessly with macOs, no need for windoze at all.

I've been running MP 5.1 with Catalina and TB3 (Titan Ridge) connected to an RME UFX+ without any issues at all - actually the performance is noticeably better than USB.

There are many guides how to get it working, you just need to follow simple steps and have patience to solve any unexpected issues...