Sounds like a mental block to me then. If you’re squatting 425lb and sumo deadlifting 475 then I cannot imagine some muscle weakness holding you back. Heck, one could argue that your sumo deadlift is low relative to your back squat.
I dabble in Oly lifting (only the clean; fuck the snatch). and I know that the deadlift and the clean are completely different movements. It took me a long fucking time to really figure this out with my body though.
What do you do differently in setting up the deadlift vs the clean?
One thing I’d recommend is that you try deadlifting light weight intentionally with a rounded upper back, or at least not concerning yourself with your back at all. A rounded upper back is safe; your thoracic spine can do all sorts of wonderful things.
Try this and then adjust as needed to your body-
Place the bar just slightly away from your shins. Hinge straight down and grab the bar. Start pulling the bar towards you as your drop your hips some more. Ideally this is simultaneous and you don’t really move much at all, but I think this is highly personal to what you like. The goal here is to get the bar as close as possible to your center of gravity while pulling your lower back into a good position. Don’t worry about what your upper back is doing besides squeezing your armpits together. As soon as the bar touches your shin just push the floor away and straighten up.
From what I understand, the mere act of getting the bar to your shin should change the way the pull occurs, and the upper back tightness you’re getting from squeezing the armpits should also be different from what you want from the clean.
The goal is to focus on four things- scrape your shins with the bar/push the floor away/back tightness by squeezing the armpits together/crush the bar. Grind the bar off the floor. You just need to find the correct initial distance between the bar and your shin to make all this happen smoothly and painlessly.
PS- if you really want your upper back to round then don’t squeeze the armpits together. Your lower back should be safe as long as you’re actually scraping the bar against your shins + you got a good back positioning from all your set-up and your hip positioning doesn’t shift during the actual pull.
Hope this helps.