Not an expert I wll say very forcefully, but if I was going full on sumo, I'd make sure I had some conventional stiff leg deadlifts in there pretty heavy too.
Sumo to me, you sort of squat the weight off the floor, then it's just grip and hip and back to finish.
But, bottom line is deadlifts are a low back exercise. At the end of the day, you have to strengthen the low back and if the full on exercise isn't doing it, then the assistance should be the hardest thing for it their is.
For me, stiffs are the hardest low back exercise there is. If RDLs are hardest on your low back for you, then go for it, but that is not my experience.
Again, just my .02
I am Anton Zdravko Martin!
Best meet lifts: Sq 150 Kg (330 lb), Bench 120 Kg (264), DL 160 (352) @89 Kg (197)
Best gym lifts: Sq 375, Bench 280 (pause), DL 385 @205 or less
Goals beyond my wildest dreams, 600 Kg raw total, 200/160/240
Hack away at anything which isn't essential. Do what you love, and do it often. Fazc.
Everything competes for recovery so more assistance is not always the best idea. miked96