Any reason you used Cast Iron? I've only ever made caramel in a non-stick pot, and it works wonders both for cooking and cleaning up. You should be scraping the sides with a silicone spatula to clean the sides. Actually for my caramels the spatula is the only tool I need, along with a candy thermometer.
I have two largish pots. One is my Lodge 5 Qt Dutch Oven and the other is a very thin walled steel one that came with the induction cooktop. I read thin walls are bad so went with the Iron.