IRC. It’s real time and an open technology platform. I have looked at Discord, which can do so much more like placing calls, but still prefer IRC.

Otherwise I still enjoy old style discussion forums. They are best if they support markdown. GitHub issue threads are ok.

Wiki page before meeting is basically what we do.

Can you explain more? Like which tool do you use for this wiki page? Or is it an internal tool? And do you use it to write meeting notes and then discuss on the same page?

If it's a discussion "too big" for slack/teams, we create a confluence wiki page to go over the details, discuss it using the Talk add-on (lets you make comments in-line) and then have a meeting to go over it.