Aside the imessage thing you probably will not notice much difference. You should use the Pixel for a few days and see if you can get along with the oled flickering that the screen shows. I don't use my 6a as a daily driver because it stresses my eyes, i prefer the lcd screens who are much easier to read.
But apart from that, Graphene OS is really working well. I also have an iPhone XR, but i wouldn't say there is anything missing and Graphene is very stable, never had something that didn't work. Their web browser is working well, the usability is excellent (if you can manage your iOS settings, you will manage the Graphene settings too).
A problem could be that you use iMessage. If you really use this daily, it might be annoying to switch from iOS to Android. If i had a service that is really only running on one specific platform, it's not worth to change the OS.
And think about that the iphones get much longer support. Graphene supports Pixels mostly for the time Google supports these, and that is only half the time that iPhones get nowadays. That doesn't make them so expensive if you really use these as long as they get supported.
So i think a change will not make you lucky. In my case the 6a oled screen makes me still use the iPhone XR as a daily driver and i plan to do this, until it's really dead or doesn't get supported anymore.
I could think that your "screen" will be the missing imessage feature and that will make it hard to use only Android.
But in general, Graphene OS is really great, easy to install or uninstall, easy to use, no real bugs and constant security updates. If i should ever switch fully to Android, then there is no other choice than a Pixel running GrapheneOS.