I was today years old to learn about https://unifiedpush.org/ (thanks to @nlnet!) and holy wow!
Since switching to CalyxOS, private, reliable, but battery-efficient push notifications has been a "choose one, at best" sort of situation and this changes *everything*!!
Matrix (Element), Mastodon (Pachli), Telegram (Mercurygram) works, there are even forks for Signal! 🚀
#degooglification