I think Discourse fits all these, I think. Where possible I use Jitsi for video chat, it’s got a better record than Zoom (not that Zoom is anywhere near the worst).
Here are my views on Discourse: sorry, no decent mobile phone experience, when it comes to editing posts, as it’s too cramped. For having live chat during my Dhamma Talks, I personally chose Mattermost Team server (after trying many possible open source alternatives). Mattermost has a decent (albeit not great) Android (or iOS) app. The Mattermost app is not quite as smooth as, say, the evil and proprietary Discord, but at least it has (currently) free push notifications which work, and it’s not crowded-in and cumbersome.
Here’s my opinion on Jitsi: I set up my own Jitsi server, and even when it was installed on a very close VPS (very low latency), the QOS rules (or in other words, the lack of “Net Neutrality”) of various ISP’s dragged down the traffic horribly, and made me look bad, even though it was not my fault, nor Jitsi’s fault! To understand these complicated, unintuitive and pernicious QOS rules more, please watch the video below, which explains it well:
There simply are no good Open Source alternatives to, say, Zoom, because you can’t wave any magic wand and make Net Neutrality be solidly established all over planet Earth, forever. I hate to say this, but if you have got to have highly reliable video chat, then you have to be willing to settle for less-than-ideal choices (at least these days), such as WhatsApp. It is the 800lb gorillas such as GAFAM who have got the QOS rules all tilted in their favour, when it comes to video chat! Tough cookies, everyone. (Yes, I’m aware WhatsApp went down for a little while the other day.)
If you can settle for push-to-talk voice memos (which I use quite a lot these days), then Signal is awesome. Sometimes, Signal video calls work great (and don’t get dropped after a while). YMMV.