> hard to imagine a faster way to make Earth uninhabitable
Manufacturing lots of SF6 (sulphur hexafluoride) might do the trick. 22800x more greenhouse potential than CO2 per unit mass.
It's way more dense[0] than air, though, so not a lot of it reaches the upper atmosphere. I don't think it's 22800x more effective when produced at ground level.
If we could terraform planets as quickly as we're de-terraforming Earth, Mars would be prime real estate in no time. Alas.