I like to glue scraps onto the inside (like the driver cutouts) to tame any modes on the panels that I imagine exist. I no longer do fancy bracing on small cabinets.the dual config should elim a lot of that. otherwise, the thing is small enough that maybe it doesnt need additional bracing? anyone can chime in here.
You built the cabinets out of gold standard materials, and it looks like a fantastic job. Bracing can be added, but I don't think you will hear the benefit, even if there is a benefit. And it's going into your home with shelves and walls and windows and floors and vents. You may be chasing stray room resonances and buzzes once you get it playing.