Hey man, I see that you are pretty tall like me. I am 21, 6'4 about 230-240. Deadlift is my best lift, and 6 months ago I was pulling 365 for a ROUGH max, and just last weekend I pulled 545 at my first meet.
First off, I only pull conventional. I feel like I just can't do sumo as well, especially with the bars/equipment at my gym. I would recommend trying conventional, as I feel like the leverages are better for some reason for people with our build. The things I focused on was being able to touch my toes, then touch my palms on the ground, then do that from a raised platform, and also work on trying to do the splits. I also made an adjustment to my form. Cues were bar sits directly over where the foot gets cut in half, feet are directly under where the legs come out of the hips, and arms grab the bar where I stand naturally with lats flared. I do this every time I deadlift, and I can set up anywhere, anytime, even if there is no bar. As far as rep schemes, I started at 315x5 for 3 sets, and increased by 10lbs each week until I stalled to the point where my 1st set could not get 5 reps. *I also did a heavy single (about 20lbs over working set) after the 3 sets. Then I dropped weight about 60lbs, and went for volume. 10 reps, 3 sets (maybe I'd get 10 on 1st set, 8 2nd, and like 6 3rd). I got to where I could do 405x10, then started deficits. Man I also did rack pulls from knee height, heavy as hell, and a lot of rows and lat work. I finished deadlift day/ back day with stiff legs, and that just seemed to shoot my deadlift through the roof.
I should add most of the training philosophy I got was based off of George Leeman videos, and his old posts on this website and some of bodybuilding dot com, but that website sucks anyway.