This is driving me crazy, and it's not even my machine.
Here's another blind guess, if you're comfortable editing your path in vi or another text editor: copy the existing path and comment out one of them. Move all of the /usr/local paths to the end.
My hypothesis is that if there are brew installations that conflict, moving them to the end will "intercept" them and allow the system versions to be loaded preferentially.
After doing that
$ source .profile
$ source .bash_profile
from your home directory.
If that doesn't work, please run
$ /Applications/RStudio.app/Contents/MacOS/diagnostics
and cut and paste down to
User prefs: /Users/rc/.config/rstudio/rstudio-prefs.json
and we'll try to parse that