Sure, you can have more freedom on the OS level, but is it really a big benefit compared to just using GrapheneOS?

With more freedom comes more privacy and security. I don’t think GrapheneOS is entirely free software, but correct me if I’m wrong.

The killswitches on the phone seem nice, but I do wonder if they’re using any proprietary firmware or something to make it switch on or off?

I think they just electrically disconnect the modem (so that you don’t have to trust its proprietary firmware to turn it off) or the microphone or whatever from power.