The politics issue has been mostly taken care of. There were a lot of toxic users of a right wing mindset in the past, and a lot of them left, while others mostly stopped commenting once a Democratic discussion was created where they weren't allowed to provoke people, and a single Politics thread absorbed most of the Trump talk and culture war clickbait discussions. In the meantime they'd already scared off a lot of left leaning users, including some that were good contributors to on-topic discussions. I think the bleeding has stopped though.
That said making certain forums opt-in might be a good idea, so long as it was easy enough to find a tutorial for how to customize the site. The way I seem to just stumble upon features that existed for free users for years makes me worry for new users. I think a well updated and visible tutorial page would be useful, probably even to longtime users for certain things.
Now with all THAT said, it's been discussed many times in this thread that the mobile experience is the bulk of the issue, and anything else we come up with to improve the place will only have a marginal effect in comparison.