If I may, here is my font subsetting tool intended for use as part of a SSG: https://github.com/vintagedave/Fontimize

Completely different use case, where it assumes you have the font downloaded already, and every new regen of your static site you regenerate the font for what’s required to render it.

This looks really cool! When I was optimizing my SSG website, the big win was self-hosting the fonts. The fonts are still the biggest part of the payload on some of the pages, though, so I'm wondering how small I can make them. I think I'll have to give this tool a shot!