> The only thing I feel like would be viable would be one shared encryption key for large rooms that just gets shared between the members via a resource-intensive diffie-helman-like message once and then after that everyone just uses the same key.

I agree on that extent. I find Telegram on this grey area which people recommend it as an alternative to Signal or WhatsApp (these people ask for group e2e) and at the same time many communities use Telegram channels and groups as alternatives/complementary to Discord. An app as Telegram is not going to please the majority and maybe this could be the case for Revolt.