+1 for unifiedpush. But I don't see how this can work in nostr, will there be an Amethyst server pushing notifications to FCM or UP? Or there is a NIP for relays to send them?
Personally, I use https://unifiedpush.org/ via https://github.com/binwiederhier/ntfy app on my phone which lets you receive and distribute push notifications to any app that speaks unified push. Dev-friendly intro for Android here: https://unifiedpush.org/developers/android/.
I run my own self-hosted ntfy server, but people who don't want to do that but prefer to support a really nice OSS project than to bend over for Google can use ntfy pro for a small monthly fee: https://ntfy.sh/app
Discussion
If I understand it correctly, at the moment nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z is running a single server that is scanning a lot of relays and then pushing to FCM.
Yep... Does anybody want to do a similar server for unipush?