I see some core team on this thread, so just wanted to say THANK YOU! Awesome job! Keep fighting for the users!
I'm totally the wrong person to offer recommendations on mobile, but so far it works very well for me, but then, I use almost no third party apps, and none of them are Play store only. My only complaint is the hardware (outside of their control).
So then what's the point of having a Play Store without Google Play services?
And even if you install Google play on your graphene phone, it is still more isolated by default. Add that to the concept of storage scopes and more permissions control (apps have to ask for access to the network) and you have a more secure platform.