Considering that no GrapheneOS dev was able to inspect a Pixel 8 (Pro) so far, it's hard to tell, but it's best if you only rely on pure hardware features and nothing based on software.
Google partially justifies the increased price tag of the Pro by locking software features behind it. When you install GrapheneOS, you erase the software, so there will be no software differences between a Pixel 8 with Graphene and a Pixel 8 Pro with Graphene. All the software features, that they announced exclusively for the Pro, will be automatically gone on GrapheneOS by default.
From my experience with degoogled phones (custom ROMs without GApps), many camera features of any modern android phone are also software-driven and part of the Google Camera app. Because the software even enhances the quality of the image on any other android, using any other camera application than Googles will result in worse pictures. Of course you will be able to install GApps and activate Google Services on GrapheneOS if you really want, but then it is really questionable why you changed the OS in the first place.
But there is some hope: For the longest time, it was impossible to run the Google Camera app without GApps installed and Google Services activated, but a few weeks ago I have read that the GrapheneOS devs managed to make the Google Camera work with deactivated network privileges and without GApps, so there is a possiblity that some features will remain. Please note that the Google Camera app the GrapheneOS apps we are talking about is the public build and NOT the new one of the Pixel 8, which is the one with all the new bells and whistles.
Now the question for me, you and many others still remain: Should we buy the regular Pixel or the Pro version, if we intend to install GrapheneOS on it? Only if you think that the hardware differences are worth for you, like the the bigger screen size, the much better telescopic camera for zoom shots, and obviously the bigger storage options, because there is no SD-card slot.