I briefly looked at running from source, and found the xcode project was a for a newer version than I have....not sure what it would take to downgrade the project...
The alternative might be to see if 3.48's calibre-debug will run the latest python code, probably depends on whether the latest can still be run with python 2.7...though thought there were framework changes as well.
I'm sure dosdude1 will come out with a patch to get Catalina onto an unsupported Mac. I saw that there had been patches to get 10.7 or newer to install onto 32-bit hardware...which I might have tried, if I had known about it. Since until a last year, I still had a first generation MacBook Pro (core duo). I was in the process of trying to find a new hard drive for it (had already gotten a new battery), and reinstall with final 10.6 (re-purchased from ebay) When, I had the darn thing stolen
Not sure if a new MacBook Pro/Air/X is in my future.....might attempt another hack instead.
But, given no support for 32-bit apps in Catalina, it might be a long time before I make that leap. First I need to work up the courage to crack my iMac and attempt a video card upgrade....and, it takes a lot since I have Cerebellar Ataxia...
The Dreamer