This is great work, but it almost makes me sad that you have to do this. I spent the earliest parts of my software career implementing full spectrum geospatial intelligence product generation algorithms for US intelligence agencies and the military. Part of the code base was software that does exactly this, but not with the historical, hand-drawn maps, but with the most current maps generated from aerial and space-borne radar sensing, as well as non-USGS data of, say, the ocean floor generated by US Navy sonar surveying programs. We could generate a map of any arbitrary coordinate boundaries of any part of the earth in seconds, using data that was at most a few years old.

There is absolutely no reason any of this code needs to be classified, as the maps themselves are not, the GeoTIFF format and tiling procedures are completely open standards, and the NGA has even open sourced some libraries for doing this kind of thing, but so much of the code that does this really quickly and efficiently, directly on your own workstation using C++ native executables into formats that can be rendered by a standard image viewer, is tied up in proprietary and even classified code bases just because of the monorepo nature of early work and the fact that development was done entirely on the classified systems because of the difficulty in developing on unclassified systems and then transferring across the air gap.

This is slowly changing, and in fact what I am presently working on is automated transfer and assurance mechanisms so more development that doesn't need to be classified can be done on open systems and work shared, but it's unfortunate to see so many wheels being reinvented when there are already really, really good tools to do these kinds of things but they are kept hidden.

We also have up-to-date indexed catalogs of exactly which tiles exist in which scale ranges to solve the problem you noted of trying to query and getting no data for poorly mapped regions. We can automatically retrieve the best data available and interpolate to the desired scale range instead. And rather than only getting the GeoTIFF, you can also get the raw altitude data intended for machine consumption, which is the main geoint use case for doing projections from sensor ephemeris to ground point.

I can see you also forked proj4js. This is of course also the kind of thing we had heavily optimized code for doing, since you have to project from lat/lon to ECF in order to project from ground to coordinates in orbit, which are ECF (we had a need to do this before the open source libproj for C existed).

In any case, it's unfortunate that you seemingly need to work for free doing this to get any attention and I hope you get a job out of it eventually. Sad to say I don't think any of the big aerospace and intelligence players would give you a second look because of a basic requirement that everyone has a degree in engineering, physics, applied math, or computer science, but hopefully a more innovative, disruptive, and agile company will give you a look. I'm not sure you'd even want the work, considering many people probably find working for spy agencies to be unethical, but I'd have hired you if it was up to me.