I'm sure you've tried or at least considered it, but you didn't explicitly mention it so: XMPP? The MUC extension has no problem with large groups (after a certain point you have to disable presence advertisement and reflections and whatnot, but that's way north of 100)

XMPP has several ways of doing e2ee, the most popular being OMEMO.

The challenge will be finding clients that supports OMEMO for MUCs (multi user chats) on each platform. They exist on desktop, at the very least.

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fedpost 2 hours ago | parent [-]

This is a non-technical group so the main concern I have is the same with Element vs Element X vs Fluffy vs ......

It's definitely tempting though, it seems to be by far the most flexible solution.

	▲	zaik 2 hours ago \| parent [-]
		It's an effect of walled gardens that the communication method is conflated with the app. But people understand well that Outlook and Thunderbird is both just email. I think this analogy helps when explaining XMPP.