is it really bouncy towing?
Best practice is to run near the tire's max psi because bias ply sidewalls are very stiff and can create alot of tire destroying heat if under inflated.
If you are only going short distances it is probably fairly safe to drop that down some.. maybe 60-65psi.
or replace the tires with some 50-55psi max ones.
I have in the past with the utility trailer hauling light load 80psi it feels like its bouncing a foot in the air on expansion joints.
with the combined weight that you specified i wouldnt run less than 75psi.
You need the 75-80 PSI or the tires will run hot and fail. At 55 PSi too much sidewall flex = heat.
The air carries the load.
he is running ~1050lb on tires rated for near 2100lb (pair)