The following works well for Obsidian users, and will probably work well for Notion and similar tools...

In general you're in editing mode. When you copy in this mode, you're copying plain text. There is no "paste as Markdown text" in Teams (though it'd be a great feature!)

If you switch to "reading mode" and then copy, you're copying HTML can be pasted into Teams.

It works pretty well. Some code fence blocks don’t seem to come through perfectly, but overall not bad.