Niche down!

Find a way to appeal to a certain demographic. You can't compete with Messenger or Whatsapp when it comes to their reach.

But you can compete on features or how you position your product.

Add features to your product that the bigger products don't have. it could be something like better tools for managing a group chat, or tighter integration with a software product that your target demographic would use.

The choice of the platform, as well the launch strategy would then hinge on the user demographic you are targeting.