I wear a tight belt and have never had my waders fill up after a fall. It sounds like the belt is too high if it hinders breathing. Seriously, does your regular pant's belt hinder breathing? That is where your wading belt should be also.
Secondly, you may have the wrong size waders. I've not saying you do, but sizing could contribute to the problem. With a wader that has an intergrated wading belt system, the wrong size could put the belt at the wrong place. In that case, use a separate second belt that is tight but positoned correctly.
I also agree that a wading staff is a must to preventing falls. The other factor is your wading boots. Have you gone to the new rubber soles? If so add, studs for a better grip. Even the new Vibram rubber does poorly on slippery rounded rocks.