Where do you get Android phones for cheaper than a Raspberry Pi (e.g. $35 for a Pi 4)?
Lol, if you have them at that price I'll take 5. More like $80-120 at current market rates, and then you still need to buy everything else that's already included in a phone.
Choose a popular, old flagship that's well supported by the community, and can be flashed with a de-Googled ROM and you're gold.
Old OnePlus devices are still supported today by the community.