I've been using Xiaomi Pocophone F1 128GB since it was released and it was 1/4 of the price of the equivelant Samsung Galaxy or Apple iPhone with similar performance. The only noticeable downgrade from those was the screen and camera. I am not sure what the newer models or prices are but if the Pocophone F1 is to go by then it could last 3-5 years without showing much age. I can't vouch if your data isn't being stolen by the chinese government though :). Lastly, I've been able to install apps like AdGuard with no issue at all so if you want to modify with your own code i don't think that would be a problem. After 4 years of ownership and countless waist high drops, the only recent issue i've had is my charging port can be a little tempremental.

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Also Quote from the Wiki about the Pocophone F1: "There is a very active software modding community for the Poco F1 and there are a huge variety of aftermarket custom ROMs or operating systems for the end user to install as they wish (after the user unlocks their bootloader)."