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Discord is pretty good in practice (and I use it a lot), but it has no data portability/self hosting and the ToS bans alternate clients. Not a good fit for a FOSS project. It's like BitKeeper all over again. Slack, etc are like Discord except worse in every way.
Messaging apps organized around private chats with basic group chats as an afterthought like Telegram, Signal, LINE, Hangouts, etc are right out.
Matrix seems to be intended to be this federated utopia, with all the complexity and moderation headaches that entails. There are bridges to other platforms, which like all bridges, randomly break and are inconvenient.
Mattermost and Rocket Chat are just Slack clones mostly intended for businesses to self-host or pay for cloud subs. I've used both. I'm not very impressed.
I'm trying Zulip and it's... overcomplicated? I can see how topics make sense in certain larger communities with lots of task management (like Rust), but not really for a project like Asahi Linux. And then there's the bad first impression their approach to auth gave me already.
Plus most of these haven't reached critical mass / appeal, except Discord and Slack (out for the aforementioned reasons)... and IRC.
And so I'm left with the idea of just having a freenode channel, with @whitequark's logging bot in it, letting people bridge in via the Matrix bridge at least until that becomes a problem, and calling it a day...
Yes, it's terrible, it's plain text only, it has no sane file transfer, it has no log integration, it's not particularly newbie-friendly... but nothing else seems to be strictly better in enough ways.
And at least I already know how it works and so do most FOSS devs. 
I haven't made my decision yet, feel free to try to change my mind.
To everyone saying Discord this Discord that: look, I'm in more Discord servers than IRC channels these days, I get Discord. I still think it's not the right option for this project, and enough people I care about would never use it.
One thing that *might* end up happening is dev chat on IRC, user chat on Discord, or something along those lines.
Telegram groups seem like a first-party feature to me, but the largest I op is ~500, and telegram still fails several of your other requirements.
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Requirements: open protocol, data portability, cloud hosted (but option to self-host).