Why would you call the Fairphone an insecure device? It has long update support and can re-lock the bootloader, which is like the one criteria ROMs like that pick as the security feature. What is your attack scenario, what your security criteria here?
microG is not a drawback, it's a proper FOSS implementation, which I vastly prefer to running Gapps in a sandbox. App compatibility has been perfect for me.
The main criteria for it to be strictly better is that you do not give root to a dev that hallucinates enemies and then send their goons to attack them. See https://www.youtube.com/watch?v=4To-F6W1NT0. It's nice that Graphene has a hardened kernel, that helps nothing if you can't trust the developer - different attack scenarios.