GitHub has an API that allows a folder to be ZIPped and downloaded. (for more, see the Notes)
And there is an Isotammi ZIPinstall addon to do installs of Addons from downloaded ZIPs.
So I wonder if Gramps couldn’t have a feature where the user maintains a list of GitHub Repository URLs (and/or local URIs) used to build a local project repository with a download folder full of .tgz files and a listings/addons-en.json (Similar to the make.py file from the Gramps addons-source repository. It builds an Addon&nbps;Manager compatible addon list).
It is important that the list is not be buried in a GUI. Being able to paste a list into (or copy from) the fault-tolerant maintence tool would make sharing a list simple.
As a bonus, that process could check that the just-built project repostitory is in the Addon Manager’s list of Projects and offer to add it if necessary.
Notes
Perplexity AI notes on the GitHub download API:
To download a specific folder from a GitHub repository via API, you can use tools like “github-files-fetcher,” which is a command-line tool designed to download a single folder or file from a GitHub repository. This tool allows you to specify the folder you want to download, making it convenient for extracting specific content from repositories. Additionally, you can refer to resources like the one provided in the first search result for more detailed instructions on downloading private GitHub repositories via API.