I have a us Samsung phone so I an unable to root and replace GMS with micro g. Would it he possible to patch apps (with lspatch for example) to use another app for push notifications? Firebase notifications are the only reason I have GMS at all still.
Some few apps like element and tusky allow for the use of UnifiedPush through clients like ntfy. All other apps either create websockets of their own (signal) use pull notifications (redreader while i still used reddit) or get no notifications at all.
It has a default push server setup that you can use if you wish. You could absolutely run your own server too. Ntfy has to be excluded from battery optimization to allow it to keep a websockets connection open. That is part of what makes degoogled phones drain so fast is lack of firebase notifications causing every app to maintain their own connection. Firebase covers hundreds of apps with a single connection rather than hundreds of individual connections.