The OS is not very relevant to the Pixel. Compare the Pixels you like that are new (GrapheneOS drops support as models become older flagships, I think for security reasons) and get that one. IIRC, currently only Pixel is allowed, because the bootloader can be opened without rooting the device.
Unrelated to bootloader or rooting. Pixels are the only phones that adhere to the device requirements that are listed in the FAQ.
https://grapheneos.org/faq#future-devices