I think it would be nice to have a despawn mechanic for grass and planks, or possibly something similar to the debug that moves bugs back to their spawn, but instead one that cleared all the grass that has been cut down and not used as it gets in the way.

Just make a weed killer that when applied to your structures disables grass. Of they prevent grow back completely some mobs would destroy areas.  Especially those infected ladybugs and their bombs.  Other mobs that destroy grass are larvae, ladybugs, infected weevils, all it takes is an aggressive mite to start a fight to trigger them. I lost a bridge over the pond because a wolf and a lady got into a Kaiju battle