You are not alone. I have a double axle trailer with house trailer axles. Brakes on all four wheels, new tires, two tire shops to try to balance.
Loaded no problem. Below 35 or above 40 no problem.It doesn"t take long for me to speed up or slow down! I have owned the trailer for some time. It was like that when it only had 2 wheel brakes and old dry-rotted tires. I carry 90lbs in each tire. Under inflation with a load is NOT good. If I were making an empty long trip on slow roads ,I might consider dropping pressure to 50#? Otherwise I just adjust speed accordingly.