How important is tongue weight with lots of weight in bed

As stated you need balance, if you get the tongue weight to light it will sway. The lighter the tongue the lower the speed that it will start to sway. I've pulled a cargo trailer that started swaying at 30 mph and after moving weight forward 3 times, got it to where I could travel interstate speed with no problem.

The tongue weight and the load in the bed are effected by how much weight the rear tires can handle.