Hi, Stef-
That looks like exactly the way you would want to do that procedure. You could download whatever package version you want, put it on a USB, say, and then use the "-local_package PATH_TO_USB/BINARY_PACKAGE" option to install it from there.
Sorry you have to reinstall everything (with dependencies? Like R???) every month.
Something to note: a way to approach this all would be to have your initial data, and some scripts, and then keep building your scripts over time, even as the AFNI version might change. At the end, get the latest version of AFNI and run the scripts from the start again, with everything being The Newest, at once. That may or may not be so practicable, depending on how interleaved things are with other software with large runtimes (e.g., FreeSurfer's recon-all). But another way to approach this.
--pt