I would have imagined that best practice was for the admin to install commonly used packages in the site library (if that what it is called= /opt/R/3......./lib).
However the local admin regards best practice in terms of performance is for users to install packages in local libraries. I can understand this in terms of keeping the install "clean" and reducing the admin work, but not performance.
I am not saying users should not have the ability to install locally. I just wondered if a new system would be better with commonly used packages already installed on the nodes.
Probably best practice is to use renv, and have a shared global library cache from which users can more rapidly populate their per project library needs.
I don't think there is a single, universal best method to manage packages in centralised vs local user libraries.
This is why the RStudio Reproducible Environments micro site lists multiple general strategies for addressing this problem. Many of these strategies use the renv package, although this not essential.