I'm no slack advocate, but you just described using three services to replace one. There will be some extra mental overhead to such a setup, even if it is not huge. I think something like mattermost carries the advantage of one service, one interface, one place for things; it's a little easier than "oh, in which did I put that?".

I wish there was something as simple to use as Discord but selfhosted. The closest thing I've found so far was rocket.chat which is only halfway done or Matrix which has close to no admin tools available.