It is matter of content, not platform. If you are looking for maximal SEO then WordPress or another CMS solutions serves you much better — and even then it is matter of content, again.
Do you have issues with Flarum that Discourse will fix? If yes then you should continue. If not, why are you looking for new alternative? If the answer is only SEO then you should rethink things 
Discourse needs basic skills of shell. After that it is only price question (plus some technical matter, but those are different thing). If you want to get one-click install or other ready-to-use solution CDCK and companions are the answer — but those aren’t cheap.