I use ntfy for two Matrix clients and battery drain is negligible.
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I never thought about using instant messaging platform... and that makes so much sense I feel stupid.
Ha that's a way too, although I was pitching ntfy. I don't know much about it, but you can use it separately from IM. Foss Matrix clients just use it as a substitute for gms.
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.