Stealing the thread a bit...and showing my obvious bias towards Ol'rs. Case in point:Mikhail Koklyaev
6'4"...I think that is taller than everyone moaning about their lever disadvantages. In this video we see him pull almost 900lbs on the DL. Absolutely no back round. He does use some straps...not too worried about that. +
I'm kinda late to the thread but I think that flat back, unrounded form is both possible and highly desirable. Also agree that rounded back lifting has its developmental purpose.
Not a fan of the trap bar DL. Turns a posterior chain movement into more quad work IMO.