This is not necessary and the lag kills it. I have a 5.5 inch iPhone now so the screen size is irrelevant on this. Make a car radio that docks your phone in landscape mode and has a car mode and that would make more sense for a good price. Sony did this for android phones and i think iOS might work with it too but it connect wirelessly i think and car play is not activated. $600 for a laggy device is not acceptable, a $399 or less iPad wouldn't even lag like that for half the price. RAM is so cheap now and they either didnt use enough or used an old slow processor. With hey siri command and a 5.5 inch iPhone this is worthless
I'm not defending the lag - I think at this day and age it is unacceptable. However, show me a $399 iPad that can completely control your cars audio system. If you want to double sided tape an iPad to your dash, well, do it. But don't expect it to look pretty. If you want to integrate your iPad to your dash with a custom fit, expect to pay a whole lot more $ than replacing a head unit compatible with CarPlay.
If you want to integrate CarPlay in a car that doesn't support it, and do it elegantly, then buying one of Pioneer's higher-end units will be most likely the best and cheapest option. Go talk to some people that custom fabricate car audio equipment and see how much they charge per hour and then do the math.