That looks great.

Would it be possible/feasible to have a "fonts" folder a level above the themes folder and - somewhat as in "languages" - search for fonts there.

Obviously, there would be a mechanism needed to download fonts (like languages) when they're not already installed.

That makes the API more complicated, I guess, but would be worth it, I think. Downloading the same fonts that are already downloaded by another theme is also not very sustainable or ecological.

In some WordPress instances I already have a fonts folder (but am not sure how it got there).