"...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.
Thanks!
When you click on one of those tags/topics at the bottom of a post that takes you to a feed of all the posts that have included those tags in the post. Community Cross Posts are meant for sharing to communities not for sharing to a new set of tags.
I literally laughed out loud. I have been here for nearly three years and never knew tags were clickable links. That's one of the funniest things I've heard all week.
Thanks!