With all due respect, we have a lot of people who tell us their form is just fine, and that turns out not to be the case. I've experienced this plenty in real life as well.... So it's not something I can assume to be true just based on your word. But that being said, if you're using a weight that is so light that you can do 4 sets of 15, perfect technique won't be essential, so it's not a big deal.
But then you said your goal was to go heavier on your deadlifts. Which is not accomplished by working in the rep ranges you're using.
I'm also not sure what your ACTUAL back problem is, it still seems nebulous. Aching in the lower back is not necessarily a problem. And that also doesn't sound like a nerve issue. I assume you know that given your background.
I'm really just confused all around. But I'm going to go back to the original question and provide a list of what I believe to be the best lifts that support the deadlift.
Rack pulls: they reduce the range of motion, and for many people are much safer. If you don't know what these are, they're basically just setting the bar on a rack, generally, with the bar height just below, or at the bottom of, the knee, and deadlifting from that point, and also deadlifting from a dead stop each rep. don't bounce the weight, set it down each time.
trap bar deadlifts. These change the leverages of the deadlift to more resemble a squat, and are less of a hip hinge. To me, these are less of a true accessory/building movement for the deadlift, and more of a replacement for folks who don't compete in lifting sports and don't need to deadlift a regular bar. You likely qualify here.
Kettlebell swings. These will give you a severe lower back pump, which is a good thing for building strength. You don't need to go extremely heavy on these. I like these to be as dynamic as possible. I'm currently training with a 35 lbs KB, and using as much hip pop as I can to send the bell up. when done properly, these should be almost ALL hips. You could use a 10-20 lbs kb and be just fine. These can be performed in the 15ish rep range and be effective. I don't like training these particularly heavy, because they are dynamic, and you're more at an injury risk going heavier.. That's something you need to avoid.
Back extensions. Use that 45 degree back extension bench thing (i haven no idea what it's actually called), and do back extensions. You could perform these with no added weight and probably be just fine. I don't like holding a weight when i do these, i think that's awkward. I do, however, like wrapping a band around the bottom of the apparatus, and behind my neck, and doing them. This increases the tension at the top of the movement, which I really like. When I use bands, I really squeeze the top of the motion for a couple seconds. At the bottom of the range of motion, the band tension should be little to none. preferably none.
I would avoid good mornings.