As a last resort, if CmdStan doesn’t have problems with Catalina (I would think more it’s likely to be ok than RStan) then you could try out the new (unreleased, and unannounced until now, and subject to change) CmdStanR interface, which can be installed from GitHub
devtools::install_github("stan-dev/cmdstanr")
There’s a brief intro vignette showing how to install cmdstan (there’s an install_cmdstan function) and then use cmdstanr to fit models from R (and obtain a stanfit object from the results if you want):
https://mc-stan.org/cmdstanr/articles/cmdstanr.html
The one important feature it’s currently missing is the ability to run chains in parallel, although you could wrap it in a call to mclapply (or something like that).