The destination civ reducing relations by -20 is incredibly harsh, especially if it doesn't immediately start going back to neutral. If it has to exist at all, I'd rather it be something like -10, +1 after every turn. Trade route destruction is a very temporary effect, and one of the only ways a defensive civ can get a commanding war score in order to end a war without changing the geopolitical landscape long term. I'm bothered a little if a trade route to me gets plundered. It's definitely not something that hurts relations with whoever did it. Morocco plundering my trade routes outside of wartime is the only plunder act that truly hurts diplomatic relationships with the AI.