You might consider what I did with my 5,1-- which was to install Catalina using the DosDude patch (macOS Catalina Patcher). Did it in anticipation of N12, and everything is running without a hitch.
You do have to make sure you’re not working with non 64 bit system level stuff (learned that the hard way, but only lost a day figuring it out 
), but doing a clean install to a new drive would take care of all of that. I was experimenting with an actual update of my (cloned) old Mojave drive, which was not necessarily the smartest move in the world, but ultimately worked out fine, and I managed to keep all of my authorizations, which may have made that lost day worth it.
Bottom line: with a Metal-compatible graphics card, you can run Catalina (and Nuendo 12) just fine on a 5,1 Mac Pro. I’m doing it right now. Well, procrastinating. But you know what I mean.
Chewy