Didn't read the whole thread but you're trying to pull yourself over the top of the bar instead of finishing with your hips.
Stop leaning back to the finish the pull. Lock the knees a bit earlier than you would conventional and bring your hips to the bar to finish the pull. No need to exaggerate the shoulders back, head back, hyperextending lumbar position at the top.
The Sumo Master has arrived. Have you done a video on Sumo Technique yet Tom? If not, you should.
Worked sumos again last night for 295X15X1. Used double overhand and toward the end my grip started to weaken so I switched to straps. Still trying to find my hips in this movement, in some sets I was too high I feel and too lower in others. Tried to use my hips more while eliminating the exaggerated lean back at the top. Anyways take a look and let me know what you see.
295x15x1 - YouTube