My favorite coding font is liberation mono, but I wish I understand truetype well enough that I could make it substitute -> with a proper right-pointing arrow, and !=, >=, <= etc. by their proper mathematical equivalents.

I've tried editing the font myself using fontforge, but without any luck...

Those are known as ligatures. Ligaturizer[0], which copies Fira Code’s ligatures into any font, may be of interest.

[0]: https://github.com/ToxicFrog/Ligaturizer