There are just so many fewer distraction events coming from the damn device. It's almost peaceful. I can forget that it's there and get on with the things I want to, or should be, doing. I've also had mini-rants before about the unimportance of email communication; having notifications about the receipt of an email is as useful as a reminder to breathe (with some edge cases that if you set things up correctly, can be managed). Email, SMS, chat apps aren't real-time communications. If it's important I'll get a phone call - and only my chosen contacts cause the device to make a sound.
(This isn't about GrapheneOS, it's about choosing sanity at the time the initial choice is requested after having the experience to know what I want and what I don't. But GrapheneOS is great, for separate reasons).