@chewie By using Signal-FOSS or Molly-FOSS you just force the Signal app to use its WebSockets-based notification fallback mechanism. But if you have Play services installed and have granted them network permissions, you might as well just use FCM for notifications. It saves battery life compared to each app maintaining its own background network connection.
If you're using @mollyim you also have a third option for receiving notifications: @unifiedpush
But since Signal doesn't support it natively, you need a mollysocket server that receives the Signal notifications and translates them into UnifiedPush format. The mollysocket server is connected to your Signal account as a linked device, but it does not have your decryption keys. There are public instances of mollysocket like mollysocket.adminforge.de
The easiest and best way to get started with UnifiedPush (IMO) is the recently released Sunup distributor app.