@daniel @masoud A big part of the reason centralized push-notifications (aka "Google Play Services") exist is for battery optimization.
You could of course just let Signal use Play Services on graphene if you like.
My experience is different (although I only have Signal, not matrix). My refurbished 7a with GrapheneOS and no Play Services at all in the owner profile lasts easily two days. (That said, it'll mostly be on WiFi while I'm at home -- so possibly the difference is that?)