I help run the OpenToAll CTF team - while the primary focus for most are CTFs, the community (>500) has really expanded, e.g., (in the slack workspace) we have active channels for N-day repros, bug bounties with internal competitions, financial trading shop talk, and hardware hacking. You wont find much OTR or blackhat stuff ( due to slack ToS), but Ive found the community loves learning and discussing all things sec.
It doesnt hit your mark completely, but hopefully its somewhat helpful
Originally we were on IRC. As the team grew, more CTFs were being played - often concurrently - so having dedicated (private) channels for a given CTF and "sub" channels for its challenges gave us a lot more flexibility.
That being said, we've considered migrating to Discord, Zulip, or Matrix - just haven't gotten around to it yet.
Please consider using either Zulip or Matrix from the get-go, else if you just pick Discord you will find yourself in the very same situation as with Slack before long.
I was in a little CTF server which got flagged and taken down, even though there wasn't anything too particularly nefarious going on. Maybe someone did upload something dumb which raised flags on Discord's end, though.
I miss having a place to hang out with other people messing around with random security projects and CTFs...