The Universal License Terms of Microsoft (applicable to Teams as well) clearly say they don't use customer data (Input) for training: https://www.microsoft.com/licensing/terms/product/ForallOnli... Whether someone believes it or not, is another question, but at least they tell you what you want to hear.

What if they exfiltrate customer data to a data broker and they buy it back?

It's not customer data anymore.

I think a self hosted matrix/irc/jitsi is the way to do it.

We've been using Mattermost and it works very well. Better than Slack.

The only downside is their mobile app is a bit unreliable, in that it sometimes doesn't load threads properly.

I would guess Microsoft has a lot more government customers (and large customers in general) than Slack does. So I would think they have a lot more to loose if they went this route.

Unless your company makes a special arrangement with them, Microsoft will steal all of your data. It's at least in their TOS for Outlook, and for some reason doubt they wouldn't do the same for Teams.