HN. Splitting off a sliver of a community isn't necessarily viable, but that sliver is alive and well on HN and posts about design that are interesting tend to get upvoted, but not a whole lot of those posts are made. It takes effort and that's in part why reddit is so successful, there is enough traffic overall that niches can flourish that would never succeed as a stand-alone website.
HN solves this problem by not having categories, which is another valid path. But to make a 'HN for designers' is DOA unless you are willing to put in a lot of time, effort and probably money to attract and retain an audience.
Pretty sure HN is one of the worst places to talk about design on the internet because its population seems oddly averse to the concept, tends to deride it as the useless whimsy of decoration-obsessed simpletons, and in general has nothing valuable to say about it except for its own obsessions with nostalgia-inspired tech aesthetics.
It would be okay if it were just that, but coupled with the classic god-complex engineer attitude blaming every problem in the world on designers (you're a "manager"? take your number, you're next) it just makes for a very cringe experience for anyone who cares about design discourse.
Yes, HN is moderated, meaning that design topics, however rare, do attract discussion. Unfortunately some of that discussion is of the "designers have too much power" variety, but you have to take the good in with the bad.
E.g., what’s the “HN” for designers?