Submaximal is powerful, but so are big singles, which is how I've broken through most plateaus for this movement.
I can remember having average form on a 170kg 1RM; I likely hitched it up at the time and it would have taken 10 seconds to lock out. But my last 1RM was 195kg (from memory) and now I'm pulling 5 of 170kg in quick succession, with no hitching.
IMO, outside of competition rules etc., if you aren't rounding your back on a max lift, it's probably not your max lift. I don't recommend everyone train that way, but it's always worked for me and my back has never felt better. Waiting for the weight that tells me it's not the right way to lift. I've failed a weight before and the worst that happens is a dented ego.