[joker99]

You‘re describing Wire. One could quibble about the FOSS part but it is open source (gplv3 or agpl). It’s used by a bunch of European governments and agencies, as well as American agencies, has regular security reviews and thanks to MLS, supports groups with more than 2k clients. Self hosting is a bit of a pain, but people are doing it.

Give it a try

[fedpost]

I would love to move to wire, but the issue I've run into is that we'd need a custom client to move push notifications off their infra and that's a non-starter for iOS users because of app store policies (thanks Apple).

We'd be happy to pay them to host but for our size we'd be paying many thousands per year which is insane for chat.