Based on cost compared to support the best device would be a Pixel 6a cheap enough to get the full experience from a device still in production and new enough to retain resale value if you decide to move it on.
4th Gen and soon 5th Gen Pixels are and will be End of Life Respectively.
4th Gen < are provided Extended Support releases on a harm reduction basis, are not advised and can be dropped at any time. They are solely intended to assist transition to a production device 6thGen >.
See our recommended devices here:
https://grapheneos.org/faq#recommended-devices
While there take a moment to read our documentation across the Features, Usage and FAQ pages.
Installation on GrapheneOS is effectively brick proof using our web installer too at grapheneos.org/install/web
If you have any further questions, please ask and I will endeavour to answer them as promptly as possible.