Full garage ? Yes.
100% completion ?
100% upgraded cars ?
I have been pondering the last two; even did some efforts in this direction. Tedious and boring.
Added a new goal : restart game on new device; again go to full garage. Doing the "old" stuff with the acquired skills and complete knowledge of the game has proven much more rewarding than grinding 10 lappers in between special events to move that 1% up in game completion.
Can recommend it.
EDIT : 2 devices all cars proved impossible for me. The time needed to get the new cars on two devices proved not within my capacities