I would like to offer some performance improvement patches for this mod. Is it located on github or gitlab or some other source control site where I could make a pull/merge request? If not, would you be interested in making it available that way?

No but you can also contact me in the Factorio's discord channel if you like, even upload small files through it. But make sure you were editing the latest version of the mod.

Sadly the amount of friction involved in that development workflow is enough to dissuade me :(

An option anyway :) This is not a work project that requires constant patching. It has basically stood without updates even over a year at the time as considered almost finished product. As a git project it would be one of the most idle ones ever existed. And to me it'll 100% be easier to make mod changes over discord. And we programmers are lazy, low effort is best effort.