I understand! I know there is a lot of hype about Discord, especially young people love it. Despite all the features, isn’t Discord just a chat software? Reading your answer, I think you are looking for a forum software (not chat) 😄 Have you looked at Discourse? It is open source, offers a modern, hierarchical structure with categories and subcategories, extensive permission systems and role management. It is specifically designed for larger communities and allows easy management of moderators and admins.

Matrix is horrible for large communities

Interesting, what issues did you have? How large are those communities?

1. Debian is always a great choice 👍. Sometimes the packages are dated, but this OS is solid and easy. Go for it!
2. I use arch btw. Because of its rolling distribution model, bleeding edge packages and the AUR.