Abandoning conventional completely would be a mistake. Your back hurt after conventional deadlifting because your form was dogshit. I had the same problem. Once I learned to pull conventional correctly, I fent soreness in the hips/hams and upper back while the lower back felt like it did not work. Andy Bolton has a youtube video containing 5 tips for deadlifting - watch it.
He says to set up with the shins vertical and right up on the bar. After gripping he rocks back onto the heels before pulling off the floor. When you feel a hard stretch in the hams, break the floor, then once the bar is just below the kneecaps, thrust the hips forward HARD.
Lifting exclusively sumo will not tax your back as much. Mine got weaker when I used to sumo exclusively. I now lift the same amount in ether style.