osm2pgsql VS mapnik

Not sure. I'd suggest asking at https://github.com/openstreetmap/osm2pgsql/discussions where someone like Jochen or Sarah might be able to give you a more definitive answer.

If you're setting up something new without any existing instructions, you will want to use the osm2pgsql flex backend as the others will eventually be deprecated. There are a number of examples in the osm2pgsql repo you can use as a starting point. Exactly what you need to do will depend on what you want to do with the data. If all you need is one type of feature like buildings, your code will be much simpler than if you're making a complex basemap.

I know Rust, but after hitting my head against a wall trying to generate Rust bindings for a templated c++ library[1] I realized I should just write this part of my project in c++.

My current plan is to read through Stroustrup's A Tour of C++, then start working on my project. When I encounter issues I'll look the topic up in Stroustrup's The C++ Programming Language.

I'm hoping someone with relevant experience can tell me if this is a decent plan, and if there's a better or faster way to learn enough C++ to write a small program that calls such a library.

[1]: https://github.com/mapnik/mapnik