I've been using https://icomoon.io/app/ for years now to pick only the fonts I like and it can also generate icon fonts from svg so that I can merge different icon sets and pick the only icons I will use in my projects.
This is definitely my go-to for slimming down huge icon packs. Font pack companies really shouldn't focus on that since there are so many tools that help developers do this as well.
Guess you don't care for the high probability that the user has already the CSS in local cache .