Deadlift Technique Update

It’s been awhile since I last updated. I’m choosing to stay with deadlifting conventional for now as opposed to sumo, even though I’m naturally more suited to the latter. I’ve done a lot of mobility work and would say that my hamstrings should be within the average range of flexibility at this point. I’ve learned how to brace myself better, and think about the deadlift as pushing through the ground rather than pulling with my back. I’ve learned to sit back in the deadlift and pull the slack out of the bar while keeping my lats tight. After all of this, my back still somehow rounds as soon as I break the floor, even at submaximal weights. I’m breathing into my abdomen and flexing hard before I pull. Even at weights around 70% of my 1RM my back is rounding considerably.

People have told me that back weakness is a possibility, but I’m bent rowing 235x5 which at 195lbs surely isn’t that weak. I mean, I see guys much smaller and weaker than me keep a straighter back on their deadlifts. It absolutely perplexes me. I tried good mornings for the first time, and even with only 95lbs for 5x5 I felt my back straining pretty hard, so I must be weak. Considering my back doesnt round at 135lbs, but starts rounding at 295lbs for deadlifts, I’ve come to the conclusion that it has to be a strength issue with my back. If its not that then I’m stumped.

One thing I should point out is that all I’ve ever done were high bar squats to just below parallel. Perhaps switching these out with low bar squats would recruit the back and posterior chain more? I feel like I am extremely quad dominant. Anyways, I’ve posted several videos below. I feel like the weight is light, but at the same time I can’t maintain a straight back. Sorry for the tl;dr, but I just don’t get it.

how does my good morning form look?

thanks in advance.