For generating multiple extensions from one github, I've been working on a packaging tool that could help.
https://github.com/SoxMax/FantasyGroundsPackager
It currently takes a directory and expects a single extension.xml file and pulls in all the files needed by the extension.xml
It should be easy enough to make it so you can specify an xml directly and rename it to extension.xml during packaging. This would let you put the extension.xml for each version along with all the needed icons in one repo and then just point each extension.xml to the set of icons it needs.