Many thanks for this conversation.
A small update, following a chat with @Lukas_Domagala: for the visual tools group, we will use both Slack and Zulip (and use Clojureverse for announcements).
As @Lukas_Domagala said, creating something friendly for beginners is important. A Slack channel could serve as a good user space for those who have a question and do not know where to go. For study sessions, dev groups and such, we could still use Zulip streams and enjoy the organizational memory.
(We can see such duality for example in the #data-science Slack channel that lives alongside the various Zulip streams on that topic. The Slack channel serves as a first entry point for many newcomers, who may later learn about relevant places at Zulip.)