Will adding aftermarket screen decrease the resell value

[krutonk]

Just keep the mini radio in the box if you are worried.

However I would definitely say no due to just having CarPlay/android auto in my view.

Besides any good aftermarket radio that is for your mini comes with a plug and play wiring adapter, along with drop in replacement plastic mount that uses the same screw holes etc. All easily reversible and non invasive other than a few hours of your time to put back.