Yeah, the Slack and message limit thing is why Discord > Slack as far as real-time chat goes for projects. Free workspaces in Slack have a 10k messages history limit so once you get big enough, you start "losing" older messages (although they still exist, and if you end up paying, you get access to them again).
There are some other options out there to address the "company walls" bit. Products like [Matrix](https://matrix.org/) come to mind; it does require a bit more setup, but you keep control over data. It makes discovery a bit harder though (i.e. you can assume that a significant % of folks already have a Discord account for some reason, and people aren't averse to creating accounts on Slack, but they might back off from engaging with solutions like Matrix because it feels like a new thing they would need accounts for).