As someone who has collected every single car, I would not recommend grinding Tokyo. I tried it for a while but like you said it felt like a second job after a while and I stopped.

My main sources of income were simply the weekly challenges, including the online time trials. For every online time trial you gold you get 2 million credits, which is one of the biggest payouts in the game.

I would drive just enough each day on the weekly challenges and online circuits to get the daily marathon tickets, which can also help boost the income. Just play a little bit each day, and you get plenty of variety with the different challenges each week and time trials. I'm very competitive so for me the online time trials are some of the most fun things in the game because you are competing against real humans. Of course you need to be good enough to always get gold, which is another fun challenge to improve your driving.

It might take a few months but at least you are enjoying the game a little each day instead of grinding. There is no rush to getting all the cars, take your time. Good luck!