For what it’s worth, I’ve found that while the RStan setup using the mainline Anaconda packages doesn’t work on my Mac, RStan via conda-forge does, at least for me. I suspect that has to do with conda-forge using more up-to-date compilers.
Bear in mind, though, that this is with an older version of MacOS, so I don’t know if that’s relevant for Catalina. That said, one could, say, install Miniconda, create a Conda environment with the conda-forge RStan package and relevant dependencies, and delete the installation if it doesn’t work. It would be interesting to see if that worked.