Yes, setting up YunoHost on a new Debian VPS was a couple of commands, and having it install Synapse and Element was a few clicks in the UI plus a lot of waiting.
However. I thought of Element as an alternative to Slack or Telegram the way OP thinks of it as an alternative to Discord. I was wrong. Element competes with IRC. This is the only platform from which I’ve seen actual groups of people (FOSS projects) switch to Matrix. I think Matrix focuses on different usage needs than Discord, and trying it with willing Discord users will be an interesting exercise in seeing what perspectives they bring and what issues that raise, but the solution to their problem will be somewhere outside Matrix, and it will be in somewhat distant future, not with the current state of FOSS tools.
Unsure about offline-first, but yes, local-first, for which a LAN is enough and no cloud is needed, is very much what I’d rather we depended on more, instead of what’s considered professional and an industry standard. Organizations requiring SaaS (as in, a network of 20 computers simultaneously becoming unable to open an app because the Internet went down) in a country experiencing war and blackouts is an additional source of stress. Free software requiring containers and gigabytes of dependencies is also suboptimal, we should aim to simplify native packaging for GNU/Linux and BSD distributions.
Another thing, programmer visions of what would suffice in bad situations are often very Western, anglophone. Software, including operating systems, eschewing localization in a quest for a lightweight footprint are not accessible or are outright unusable for the majority of the people in the world. Please take care to make your software possible to translate, including documentation. Best if the focus is on tutorials and handbooks; manpages are less of a priority.