Matrix is good as a LiveKit client who support active call, like a phone call, and good enough for passively share screen (no remote input supported for now). Not good for chat.

A good FLOSS internal chat is Zulip, a passable one is Mattermost (who is better for many things, but being "Open Source Enterprise" is largely a no-go IMO.

Zulip+DocMost are Slack+Confluence. Zulip support calls as well, but via Jisti and they aren't much nice IMO.

I know that Mattermost came with Gitlab, but didn't like it at the time (but that was like 6-7 years ago, so it might have been better). We were somewhat spoiled with how good Telegram desktop application was/is, so at the time didn't want to move. However, as the company grows and its processes become more complex, a simple chat application is no longer sufficient. We need a more integrated collaboration platform that connects seamlessly with Jira and other company systems, supporting our organizational structure, workflows, and policies.

Will look into Zulip, thx.

If Zulip and Mattermost are in the running (no E2EE), Fluxer should be as well. Their client is a super slick discord clone, and it's very easy to self host. Federation is coming soon as well.