Reddit: Dying, not owned, but more importantly, it's bad at community.
Discord: Decent for community but seems to scale extremely badly with more content. For example, you can't give users their own space. Forums and threads become a mess.
Discourse: I can't put my finger on it. It works for support but I haven't seen it do well for community. The system with badges, Regulars, Leaders, etc seems to basically be designed to automod strangers to have power over one another.
SMF/phpbb/vbulletin: Cozy, but feels dated, especially without mobile support.
Invision Community: Actually seems good but also bloody expensive with the lowest amount set at $58/month?
Any other suggestions? Anything that I'm misreading here?
Yes, that core may attract people who never have and never will participate in the offline core and those people may dominate numbers (HN is a good example).
Online tools reinforce and amplify communities, but r/justinbeeber isn't bootstrapped. It exists because of the meatspace component.
Stackoverflow exists because people write code.
Facebook exists because people have offline lives...Facebook brings up the important point that there is no "Facebook community" all it's many communities have real world basis.
Anyway, "writers" is too broad. Experts will commune with other experts when they aren't busy writing and the platform will be dominated by people who like to dominate online communities (just like HN).
For what it's worth, if you don't care fifty-eight bucks a month that might signify your commitment level to the project. Or not.
Good luck.
Once you have built some traffic and have actual users. Only then move to a paid solution such as Invision or vBulletin.
And the engine behind Lobste.rs: https://github.com/lobsters/lobsters