If your YouTube channel was the galvanizing "complement", it may be that the Discord's usefulness shifted when your channel focus did. You could potentially find ways to better complement the new focus of the channel and see if that brings new excitement to your Discord. (Are there enough channels for your shifted focus? Are you posting your videos to them? Do people have things to say about the new focus?)

Most of what keeps a Discord active or not is content and having things to chat about. You list things in an order that suggests the chat started to slowly die and then your channel's focus shifted, but maybe it was the other way around and not as much as of the audience that had found their way through the Discord followed you through the channel refocus as you expected?