xtb VS awesome-python-chemistry

Have you tried using GFN2-xTB (https://github.com/grimme-lab/xtb)? It is essentially the fastest method you could use, after forcefields, but is designed as general-purpose tool. I haven't tested it with Zn, but it's worth a try.

I would recommend getting xtb (https://github.com/grimme-lab/xtb) and exploring some possibilities. The calculations are lightning-fast, so you have a quick feedback loop and you can quickly test different ideas. You can look at recent publications to get an idea of what people are doing.

the xtb and dftb programs are actually excellent example of well-written, clear, modern fortran codes. I've learned more from reading these codes than from any other resources.

Also I'd take a look through https://github.com/lmmentel/awesome-python-chemistry because I have a hard time believing this isn't a "solved problem" where you just need to find the right existing library.

This list of packages is probably a good place to start. It's a little unclear what your dataset looks like but at a guess you're going to need to parse molecular formulas and perhaps structures (you'll need to convert those into some machine-readable format like InChI). Sounds like you also need to consider molecular weight so you'll need a mass calculator if you don't have those values; you could build your own or leverage one of many available.