You say ttf and otf are the same, but that's not true, one utilizes quadratic curves, the other cubic curves. Also one is patented, the other isn't (most notably its bytecode vm).
Also, you say curve and spline is the same. They are not. Spline is a series of curves where you don't store the starting points, because those are the same as previous curve's ending points. So a spline requires less space to store than a list of the same curves, hope this makes sense to you.
I've also implemented a very fast font renderer, which supports bitmap, pixmap, quadratic and cubic curves, even mixed. I took a different path though: I've designed a very efficient font format, and provided a tool to convert ttf, otf, ps, etc. fonts into that format. This way the computational heavy parts are done beforehand, and the renderer can be small and fast (it's just a single header library, with source of 32k).
https://gitlab.com/bztsrc/scalable-font2
Using GPU for rasterizing is a good idea, but my font renderer is extremely portable so it's deliberately dependency-free and uses CPU integer arithmetic only. So it had a different design goal than yours, but still, it might be useful to study and it might give you ideas on further optimizations.
Cool library and CPU integer arithmetic only is interesting.