If you don’t mind self hosting Mattermost is a pretty great replacement. Comes with some neat productivity tools and integrates with a lot of services including Microsoft stuff.

You can even have a bridge to Teams so you can set it up for the sort of nerds who appreciate full Markdown support with syntax highlighting.

To sell it to your boss, just say “If Teams is down, how will we get it working again?”

My last company went to Zoom, funny enough, months before COVID hit and we all went WFH. I was the administrator and was thrilled with it. We used Google for identity management, clicked right in with some simple setup. Not sure how it plays with your MS ecosystem, but I'd bet money it's every bit as easy to tie your MS accounts to it.

We had a few clients and vendors that would suck us into Teams and Google meets and they fucking sucked. I honestly can't remember a single complaint I had with Zoom's software. Tech support was superb at the start, dropped drastically once they exploded in size with WFH and COVID. No idea how it stands now.

Google Chat is... acceptable. But its still invasive.

I always loved slack. Unless something changed in the past few years it's great.