A wiki is useful for capturing historical context, and for knowledge sharing. As the one stop shop to refer to. And honestly, kudos to you for thinking about that at an early stage, most companies only start to properly consider disseminating information once many habits have been established and it's harder to train people. There are multiple tools available for this, Discourse, MKDocs, GitBook, Notion, Confluence, MDBook. Some are open source, others are not. It just kind of depends on the set of features you need.
For daily communication, most places I've been in used Slack. I've seen a few use other IM chat options, some self hosted, some not. This is less important IMO, at an early stage, as it should never serve as your main historical communication lookup. As such, you should be free to switch over with relative ease.
For daily communication, we're all live on a discord video server during working hours so communication there is instant. We're supplementing discord with slack to enable asynchronous communication.
For roadmap & technical management we're already using Jira. Maybe Confluence would be good option?
Because there's no tree-like hierarchy, it's quite difficult surfacing information unless you know exactly what you're looking for. And while some people will know what they want, I find it beneficial to make everything as visible as possible, since more eyes means it's less likely for documentation to go stale.
At work, we've currently trialled a multi doc site using MKDocs, with different topics, published into GCS buckets, and sourced from a git repo. mdbook and gitbook offer similar strategies.
Discourse is similar to stackoverflow, and has the advantage of people being able to ask questions, but it's not the best medium for documentation.
I think something light like mdbook might be the right choice to start with, it wouldn't tie you into any particular solution, as your documentation would live in markdown, but would be consumed from a nicer, more polished UI.
This, of course, assumes you're all familiar and comfortable with markdown, but I think the best solution is to try different tools, so the more portable your actual docs are, the better.
Notion is also a very good candidate, and IMO, a better choice than Confluence, but similarly, discoverability isn't great
Leveraging markdown to enable service agnostic documentation sounds really interesting...