You're selling something that isn't a product. Standalone chat is a feature, and you won't win against a full product offering unless you drastically reduce your prices. You need to get acquired (by Google or Facebook), because I don't think you can innovate an office productivity suite fast enough to compete. And that's clearly the enterprise puzzle piece where chat fits into nicely.
Microsoft isn't being anti-competitive here. Out of all the FAAMG, they're the one that least resembles a monopoly.
You're struggling because you are selling chat as a service. Something that has been free since IRC and AOL Instant Messenger. Something any fifteen year old can write with web sockets. You built a pretty (through opinions vary) version with access controls.
You had a chance to build a moat and make acquisitions to fortify your position, but you didn't do it. You invited competition from Microsoft and instead of doing something, you rewrote your Electron layout six times and made a WYSIWYG that everyone hated.
You had four or five years to do something. You had a chance most startup companies dream of. But here we are. You squandered that chance while you reveled in hubris.
You're going to get eaten, and there's not much you can do about it now.