Your other points are great low-hanging fruit performance-wise, but replacing bootstrap in a site that has been fully designed around it goes well beyond "very little effort."
Could something like purgecss be effective with removing unused bootstrap classes?
I've never tried setting something like that up but Bootstrap itself is extremely modular and you can be selective about what you want bundled with it, so that's probably a more effective starting place: https://getbootstrap.com/docs/3.4/customize/