As far as a sub, I'd say just the one. You can pin threads so they remain visible and appear at the top of the sub easily. With a little planning up front you can structure the comments to sort the "World Building" thread into things like "Put replies relating to race comments here" and "put replies relating to geography here." It's not quite as powerful as a wiki or a structured forum, like this one, where discussions can be more carefully ordered, but it is, I think, a more popular and probably more vibrant medium for anyone younger than fifty. Discord is probably more useful than what I use it for, but I've generally used it for voice/video chat with my gaming group. The dice bot and ability to post XP rewards, character sheets and the like, but it strikes me like a commercialized IRC.

I'd say that either Reddit or the Forum are the preferable platforms as asynchronous communications, longer responses and the more measured pace nurture more thoughtful and, generally more genteel, interactions.