With regards to choice of platform, below is my experience with tools that I have encountered "upstream":
1. Slack - it's familiar to many in the cloud-native ecosystem. Unfortunately it costs $$$ if we want to retain chat history for more than 90 days.
2. Discord - a key differentiator is that Discord servers let you create an onboarding experience/tutorial, which is very useful to new contributors. I am less sure on costs + data retention policies.
3. Matrix - a true open source alternative used by Fedora + other upstream communities. Can be thought of as a successor protocol to IRC, with an ecosystem of supporting clients, servers, and other components. Matrix.org runs a public server instance.