I feel like CRAN should be used for packages that are expressly made for others to use, and with effort put in to the documentation and vignettes.

If you’re making a package for a small team or aren’t pushing it to a large audience then just keep it on a GitHub repository. It is almost as easy to install from GitHub with devtools as it is to install.packages().

Exactly. I made a small package for my lab and just put it on GitHub. My guess is that many academics don't know that you can install packages from other sources than cran