Please take this question as coming from ignorance rather than a challenge: How would anyone know that you are using Graphene without you telling them? The Pixel line is still used to run stock Android, and the Graphene OS UI is similar enough that a casual inspection shouldn't raise any flags.

If you're concerned about scanning your phone, consider a physical boarding pass. When you land in the US, turn off your phone and put it in your backpack/ carry-on or whatever.

I'm not in the US, but I assume graphene is seen as communist witchcraft over there. Lineageos is probably much more lowkey. A googled phone would not raise any eyebrows at all.

I hate google, but if you are worried about it, a googled 2nd phone with a new sim/number and a new google account wouldn't be the end of the world, especially since that set up could be temporary, tons of phones can be lineageos (my 2023 motorola g84, and 2018 sony xperia xz2 for example)