I’ve updated the Windows installation instructions to avoid most of the common issues (seems to have the done the trick, given the reduction in support posts recently). I haven’t had time to properly run through the Mac install instructions yet (since I need to set up VMs to test properly), but will be getting through that in the next couple of weeks.
In the interim, R4.0 tends to play best with rstan across both pre-Catalina and Catalina OSes. Stephen Martin had a great writeup of installation/setup instructions in this post as well: R 4.0, rstan, and you