Don't get an older phone, between the Pixel 6a and future 7a having such a low price to entry along with 5 years from release support it doesn't make sense to use a 5th Gen < device. Better ROI on current production devices.
6th Gen + recommended.
Our recommended devices from the FAQ:
https://grapheneos.org/faq#recommended-devices
Be sure to check https://endoflife.date/pixel too.