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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.
Zulip was created at @recursecenter to solve their particular challenge, and was later turned into a startup, acquired, open sourced, and rebooted. The people at @zulip seem generally quite happy to hear what they can do to improve.
In my experience at RC the topics work quite well in practice. In IRC it often happens that there ar multiple conversations happening at the same time, and it becomes a bit of a mess. With Zulip I find it's easier to have parallel conversations going on.
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Requirements: open protocol, data portability, cloud hosted (but option to self-host).