I share a lot of your concerns about Flarum. It’s sad, though, because I think the world of forum software has room for their vision, which is a very simple core, with all functionality as add-ons, so that it is extremely customizable but also every implementation can be very “lean”. I get what they’re trying to do, I just think they’re not doing a very good job of it. 
Anyway, glad you’re on Discourse now. And since you were looking at Flarum with interest, perhaps you can help the Discourse community and team work to incorporate some of the things that Flarum does get right (mostly design/layout). I’ve already contributed to a couple of topics discussing these things in more specifics:
Please check those out, vote and contribute support, ideas, etc. if you can!
Bottom line: Discourse is the better forum and platform, but it does need some work aesthetically, or at least some options to look and work a bit differently (UX-wise). There are some better models being shown, and many, many newer tools are going with e.g. persistent sidebars (Circle, Forem, etc.), so clearly there is something to this…