Okay. Thanks for all the information! That is very helpful to me. I'll take a closer look at the r-app cask. I had not used it before, but it seems I should try it now (sounds like brew cask uninstall r-app can cleanly uninstall it, which will make my happy enough).
To be clear, I definitely hate building these binary packages by myself, and would love use the CRAN binaries, if only all binaries exist. The missing RGtk2 binary has bothered me for years. I don't understand why CRAN has to provide a homemade version of Clang. There must be a reason, but this makes things hard to automate (and I hate editing ~/.R/Makevars).
Regarding the macOS builders on Travis, the extra burden I brought is quite minimal (often none). Packages are rebuilt only when newer versions appear on CRAN. And I don't really have a large number of packages in this repo (less than 30). So most of days, the builder only checks for new versions and exits.
Once CRAN becomes compatible with Homebrew, I'll happily tear down what I did here.
