Otherwise depend on what is called a “non-puppeting bridge”, self-hosted bot that reposts messages on behalf of itself. Problem is, reactions and other multi-agent interactions won’t be a subject for bridging. If you absolutely need those, you have to host a Matrix server (like optimized, but unstable, Conduit or the hungry Synapse) or partner up with another server owner to activate your appservice…
A lot. Of. Downtimes…
If you want to make a set of private rooms that you want to be available at all times, I certainly don’t recommend matrix.org.
I don’t use neither of them. What I liked specifically about Matrix is the detailed events system: I’m just not sure that kids might find it useful. 🤭
Sure. Federate with another trusted server, but probably not the general one - you might want to avoid that.
Sure, just find a way to self-host Conduit or Construct for Matrix and don’t federate with outliers - it would work like a charm for internal communication (disclaimer: these light-weight servers are in beta stage, unlike the resource-hungry Synapse). Wait, NextCloud and Revolt have implemented chats, which are encrypted and open source already? As such, I don’t observe any special benefits of Matrix for your use case (unless you’re gonna do bots and stuff). 🤔 You have a lot of choices! 😉
Disabled presence can be an advantage to some privacy-conscious users, so a case for matrix.org can be made for them - especially if they don’t chat much anyway.