sorry if this has been addressed before (just getting back to SForum after a loooong break) and i couldn't read through the whole thread
I have been deadlifting for a long time. To improve form, I'd suggest taking videos and showing them to a coach if you have one (there are some decent internet coaches who do this) or at least looking at it yourself. Leverage is key. You want to stay behind the bar, not over it. A little bit of an exaggeration but one way to think of it is if you drop the bar you should fall back on your butt.
Drive through your heals in the first part of the lift, keeping the bar as close to your shins as possible (e.g., dragging them up your shins). WHen it's above your knees, the key is to think of pulling your knees back and driving your hips forward (squeezing glutes). Think of it like a scissor jack, and wedging yourself under the bar. Many beginners make the mistake at this part of the movement of completing the lift by pulling with their lower back. THis is somewhat dangerous and will definitely limit your results. The last part of the lift is about hip thrust, and glutes, not lower back.
while straightleg deads and other lifts can improve your lifts(depending on the sticking point), i don't think straightleg will improve your form. In fact you have to be careful about the opposite as it can "teach" you to get over the bar rather than behind it
other exercises to improve your DL are dimmels (straightleg but lowering to just below the knee) -- good for improving lock out, and deficit deads which (like RDL or straightleg) are great at improving explosiveness off the floor. You can also do straightlegs with dumbbells which adds a deficit component because the dumbbell grip will be closer to the floor. I'd also recommend incorporating speed work
Personally, my "normal" work focuses on 5 rep sets, and every so often 3 reps. Not a fan of higher reps on this (at least not on a regular basis)
There are a lot of other finer points, but highly recommend this into everyone's strength training workouts!