You're in a similar situation that I found myself in.
Your back arching on the deadlift is probably a result of a lack of core strength and glute recruitment. Your erectors compensate for the weakness. You probably set up trying to get a big arch, but as you pull, your hips come up before you're able to get the bar to break the floor.
On the squat, you probably have poor hip mobility. Your lumbar spine probably compensates for the lack of mobility and you consequently round your back.
You probably won't like the advice I'm about to give you: Unfortunately, you've probably "memorized" improper movement patterns. If I were you, I'd stop squatting and deadlifting completely. Focus on core stability, single-leg work, glute recruitment, and hip mobility. Personally, I created a squat progression that took me from a squat variation where I held a plate in front of my body, to zercher squats, to front squats to a box, to front squats and back squats to a box, and finally back squats. I also created a deadlift progression that went from suitcase deadlifts so one-arm dumbbell deadlifts to 2-arm dumbbell deadlifts to sumo deadlifts and rack pulls to conventional deadlifts. The focus was on re-learning the motor patterns on easier movements and gradually "learn" to use my core to stabilize and recruit my glutes on the movements.
It will probably take you a few months of training to fix your imbalances. However, once you do, you will probably find yourself improving at an incredibly fast rate. I'd advise you read all the Mike Robertson articles on this site, as I'm sure they'll really help.
Your other option is to continue training like you do, except to add exercises that people suggest. If that's the case, you're just waiting for an injury that will totally stop all of your progress and set you back big time. You already have some imbalances and movement deficiencies. Simply adding some good mornings won't fix that at all.