The whole tuning menu needs to be changed to streamline things and decrease time wasted in menus.
Everything to do with tuning should have a checklist option and an on/off switch for animations. You should be able to go down a list, check the boxes of parts you want and then hit "Buy All" or something like that and be done. Scrap the GT Auto and put the oil change, chassis/engine refurbish in the Tuning area and bring in a Livery Editor for painting and aero and cosmetic changes to the car.
On top of that, you should be able to enter a race lobby, go into your garage, and hit one button to return any car to stock condition, or have a 4th tuning profile that remains at stock regardless of the mods you make. As it is now, you have to either retain cars as stock to race without tuning or hope that you don't make any reversible mods to the car if you still want to race it spec.