I am planning on doing something very similar with my bike. You seemed to have plenty of room under the plate for the tire to clear even when bottoming out. Did you bracket bend when you hit the pothole causing the plate to catch on the tire?
Just brain storming out loud, really. My state has passed a law that all license plates must be mounted horizontally, so mounting the plate off to side isn't an option for me (vertical is okay looking, but side mount horizontal just wouldn't look right on my bike).