Honestly, I think the answer is 'keep the site/community very niche'. Since the more niche the topic, the more the community both tends to stay civil and stick

Honestly, I think the answer is 'keep the site/community very niche'. Since the more niche the topic, the more the community both tends to stay civil and stick to the original vision. The somewhat more general gaming community I used to run devolved into chaos a couple of years in, and eventually the visions of myself, the staff team and many of the actual members all diverged dramatically, leading to almost complete collapse and no end of drama. On the other hand, the much more niche site I currently about a very specific series has stayed mostly drama free for the last decade, and works exactly as I'd want it to. Probably in part because the only people who join are those who actually want to discuss the topic in question.