I’ve been active in a Discourse community where we used an external chat provider, then switched to using Discourse for chat. The experience between using the external chat provider and using Discourse chat wasn’t all that different. But, if I remember correctly, authentication, usernames and avatars were synchronized between the chat provider and Discourse, so there was a sense it was all one community. As Robert noted, the possible difference in usernames and avatars between your WhatsApp community and Discourse could actually make it more difficult to create a sense of community. Also, some of your Discourse users might not want to signup on WhatsApp.
Chat is great for off the cuff discussions. I suspect that it can be easier to form deep human connections with chat than with formal discussions. Chat isn’t really writing though, and you’re forming a community for writers.
I’m not sure that you can deliberately create group cohesion. Maybe the best you can do is create conditions that help it to occur. I’d start by trying to identify some problems that your community can solve for its members. My guess is that what writers want is an audience, feedback, and motivation to keep writing. If that’s correct, I’d be inclined to try to meet those needs through regular Discourse topics. Enable chat on the Discourse site, but leave it up to the community to figure out how they want to use it.
Writers and creative types of all sorts might need some encouragement to pay sufficient attention to what others are doing. The best way to bring that about might be to be the change you want to see.
It might be worth looking at https://forums.nanowrimo.org/. They’re particularly active in November, but seem to get a lot of activity throughout the year too.