I finally figured out why my filler kept cracking on quarter panels

That Florida humidity is a killer for sure. One thing a lot of guys miss is the temperature of the metal itself. If that Civic was baking in the sun all morning, you're putting filler on a hot plate. It'll kick off way too fast no matter your mix. Try parking it in the shade for an hour first, or even wipe the panel down with a damp rag to cool it off. Makes a huge difference in how the filler lays down and cures evenly.