I tried it and it was fine while it worked, but eventually I had to go back to regular Signal because Molly's updates did not follow Signal's closely enough, and at some point the server code changed enough to where I was unable to use it for an unacceptable amount of time (after checking, it took them weeks to update). Something to keep in mind if you're not using a custom server.

Molly is a security-hardened Signal client only on Android for people unfamiliar. They went through a period of not updating (there were no security updates during that time afaict), but now releases should happen faster on top of Signal.

In Molly there's three options. Google Play Services, WebSocket, and UnifiedPush.

I use the WebSocket and Molly has used >1% of battery since the last full charge so it doesn't seem like play services would improve battery but maybe if I had more apps depending on it..

Google and Apple can't see the notification content but they can see metadata. If you want metadata privacy you should use SimpleX instead.

The native Signal android app delivers notifications just fine without Google play services on my degoogled android.

I was using Silence from F-Droid for a while back in the day because of these issues, but the lack of interop and needing to make everyone move again soured me on the whole thing. I would rather just get people on XMPP or Matrix and not use some sketchy phone-first app at all. For SMS I use Fossify Messages, which I think was a fork of QKSMS. I don't use SMS as primary or sensitive comms, only as needed. Same as email, basically, but less useful.