What I think would be the most practical is twofold:

1. Anything that's conversation-based, like discussions around features and general ideas should probably happen in here, because it's a great opportunity to just play around the interface and try things out.
2. Anything that's more technical in nature or practical in terms of the codebase should probably happen in source code management software. In our case it's Gitlab. This way we can avoid repetition and make sure to keep track of every problem more efficiently.

Let me know if you have additional thoughts on this. I'll send you an invite to Gitlab by email. :)