Flying higher is not effective for the drop. The flame retardant will dissipate and not give you the required coverage.
I’ve done this mission in C-130s when I was in the Air National Guard. It can be very dangerous. The effective drop altitude is between 150 to 300 feet AGL. Sometimes you’re dropping in a narrow valley with trees above you and not far off your wing tip.
So, sometimes you are below the trees on the ridgelines. The fire can create a fire tornado that obviously can lift all kinds of debris into the air. The burning tree sap can also explode sending shrapnel of bark and other tree materials into the air.