Karting is obviously super tough on the ribs. As you’ve found out, just driving can hurt your ribs. I’ve got a great fitting seat, great padding, a good ribvest and I’m fairly fit and have been racing a long time and I still get some pretty beat up ribs from time to time.
Building up core strength can help a bit so you aren’t getting throw around in the seat too much. Honestly, your seat should be pretty snug on you around the rib area.