I have no recommendations for versions. Depends in your case.

Although I'll mention that some people don't like that Pixel by default doesn't include the bottom menu on the screen, and uses other swiping features on the screen to do the same thing.

However in your system settings you can change the UI to gave the navigation menu at the bottom of the screen similar to Samsung.

I've got a 7 and works great

Graphene lists all the pixel models they support and for how long. https://grapheneos.org/faq#device-lifetime

Finally had to ditch my 3a this month because Google is psychotic and it's too old for graphene or lineage. Sucks bc the phone still works great. I've temporarily got an 8a running graphene, and am loving it, but will be switching to a 9a shortly for non pertinent reasons. It is such a weird feeling of relief to be on a secure OS.