Developing something like Babble from scratch alone would cost thousands of dollars if directly commissioned due to the complexity of the interface customisations. Even more if it were standalone and required user account admin. If you have that kind of budget then great, but it’s a big project.
Babble is pretty convenient? It’s sitting in the discourse app itself. No need to administrate a separate set of user accounts because it is fully integrated. Don’t underestimate how annoying that would be.
You could look at Mattermost or any of the usual chat services. Mattermost is a good choice as it is open source and easily self hosted. Separate app on phone homepage too.
Look at something like Auth0 to maintain a single source of user account so you are not having to administrate in two places. Pavilion could help you with that.
Another good option is Discord. There are existing plugins to permit Discord logins and sync user roles. Those could be customised.
Support topic because a lot of this discussion is talking about existing alternative solutions as opposed to a clear specification. Don’t worry about it. The mods can move it if they decide to.