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At this point, I think pixel 6 is oldest you can go. But really, it’s about your budget. I am hearing people still using pixel 4s.
With that being said 9 has better battery and faster charging and that has some value.
Whatever you do, consider open box, used, or refurbished because fuck Google and sundar the creep.
There is some risk there no doubt, so if you need 100% success and warranty something to do consider. New has best protection.
However, from personal experience and others talking, it is 95% the way to go. I am sure one day I will get a lemon but all the money savings will cover it for me as of today.
I generally start a lot of my purchases now on second hand markets, preferably local. Deny these parasites profit one transaction at a time;)