Finished the game and I still only have 3.6 million credits. How are we suppose to grind this hard??

As the title says how are we supposed to grind this hard ? I play all the time and I've completed the game like 100 percent finished and total earnings ever is only 10 mil. I want to buy a magen civic but it's ALWAYS 20 mil How tf am I suppose to earn enough credits to buy this single fucking car??

EDIT as of today I now have 4.9m from cashing in months worth of earned level wheel spins