Google requires you to use their infrastructure unless you want to have significantly reduced battery life.
There is no energy efficient way to listen for notifications unless you use Google's solution that they baked into the kernel.
There is no energy efficient way to listen for notifications unless you use Google's solution that they baked into the kernel.
It isn't much to ask, actually, such services are pretty easy to create.
What is the downside of using Google's? You can E2E encrypt messages and using firebase for notifications is free.