Just remember that anything you use thats not FCM based will be second tier to FCM.
There are OS level restrictions that FCM can bypass that you won’t be able to with any other service.
You can ask users to disable battery optimizations but even then, they’ll be intermittently unreliable, and you’ll ultimately end up in situations where they won’t receive them until the next time they open the app or hours later, depending on the manufacturer
And in case it wasn’t clear, you don’t need to be a IM app to use FCM. That’s not it’s only purpose.