Discord would be unpleasant for something like this with so many participants. It's a similar reason to skipping out on Hangouts, Zoom, etc. It forces synchronization, if you're not online during the discussion you're effectively barred from it. It can be very hard to catch up and very hard to respond to any particular thread of discussion. Discord is also, by design, essentially single-threaded. You can reply to specific comments but it's still presented in an interleaved format which makes tracking difficult when multiple threads of discussion are occurring at once.

If the discussion is light, it's a non-issue, but with 300-800 (per eatonphil's comment) it's likely that it will not be light.

Google groups is a forum but discord has horrible historical retention.