After a week of being temporarily tied to trees and bushes, I finally got around to anchoring the antenna a little more securely using some eye screws. In the process I was able to raise the center of the antenna up another couple of meters which might help things.
One arm of the antenna is weighted down with a 2lb weight and floats up and down so that it can move with the trees. The center and other leg are tied down with enough slack to handle windy days. I may change them so that they're just weighted down as well rather than tied off.
The coax got routed through the crawl space so I won't have to worry about running over it with the lawn mower. It runs up the side of the house through another eye screw to the antenna feed line. When the radios get moved into their permanent location in the office/shack, I'll look at some floor or wall connections for the coax.