Ding ding ding, I think you hit the head of all the nails there.

I don't use Discord for anything serious but it's just very convenient to join any Discord group with a single click since I'm already logged in there.

I think we need to put aside the security practices of Discord (it's not like the discussions tend to be sensitive in any way), because I'm not aware of anything that comes close in ease of setup and user onboarding, at this point. Setting up Zulip or Mattermost, paying for hosting and then managing it all is quite a hassle in comparison.