I was in the same situation - macpro5,1, 6 cores, SSD for system, 18TB total HD space.
If you’re comfortable reinstalling the system, I would highly recommend you install macOS Monterey using the OpenCore Legacy Patcher method. I did because, although I don’t run Roon Core on this macpro, I do curate my library there and did not want to change this. I was a bit reluctant but it all works amazingly well.
There’s another IMPORTANT advantage of doing this if you run your core on this machine: Microsoft’s native .NET library is supported on Catalina onwards. What this means is Roon Core will run MUCH better and faster since it will not need to rely on mono libraries (Roon is written in .NET).
Check this out:
