You can run discourse in a container and the only real hassle is publishing your own version of the discourse app so you can get push notifications without having to pay them.
Not in my experience.
We spent a nontrivial amount of time on running and maintaining our discourse instance - dev time that is very costly as a small company. Might be different at Posthog level of investment of course.
That was about three years ago, so maybe running discourse is much easier now.
Zulip is also available as a container.