"...the plan is for them to not show up in the UI as a cross post on topic feeds."
I am still not clear on what this even means. The difference between communities and tags will take some time to be demonstrated to me, I reckon.
One worry I have about communities is that it will definitely strengthen echo chambers, and this will prevent understanding and tolerance from growing between communities. I don't know of anything that will more strongly counter that than cross-posting, so I am very happy to see you are working hard on it.