The small amount of mods is pretty nice per each car. But, to make bigger impact maybe try something new and more difficult to challenge yourself?
Really like the bg of that bmw, but it does not seem to fully fit anglewise.
Citroen is ok and simple, but the whole thing really lacks contrast to make it pop more.
On VW the black change is well done, but it is easy to do from red to black. The wheels are not installed correctly and would not work. The angle and how far it is showed in there is not correct because it would not fit under there.
On those smarts, the version 2 is better. Ok, the dramatic light reflections at the hood are not completely correct, but looks hell of a lot better than that compromised attempt at v1. Bg fits angle etc. wise well here aswell.
On proper and common note, I would suggest trying different stuff more. On most of your chops you take a original picture, make minimal mods and then put it into another persons photo (well lol that is majority of chopping) but for example I am not sure how that speedhunters photographer here would like it. Not sure whos work it is more here to be totally honest.
Keep practising, level of your works keeps improving but still way to go like everyone else!