After more reading and testing code, I’m getting a better picture. Not quite there yet. But I did find a project called JuliaGeo which I think I can contribute to. I could add a few more transformations like Lambert Conformal Conic and a strange one that seems to be used only in New Brunswick - that’s in eastern Canada. I had to convert a lot of DEM data from their topo database to UTM. The project is a little short on ellipsoids too, but then who uses an Airy 1830 ellipsoid anymore, except for some historical surveys in Britain.
I got a lot of good material out of a text called “Map Projections - A Working Manual” by John P. Snyder, US Geological Survey Professional Paper 1395, fourth printing 1997.