The other solutions here are great and full of detail but skip over one simple piece of advice.
It’s relatively easy to 2 star each of the challenges in each chapter, you should do this first and unlock all 40 chapters. If you do this you will have 80 stars and maybe some more as it’s possible to three star some of these on the first go.
Once you have your minimum of 80 stars there are many that can be three starred by taking short cuts or thinking outside the box.
Example one. 3 starring the tunnel dash in stunt driver seems impossible until you realise you can miss most of the jumps and gain a couple of seconds.
Example two. The project Gotham chapter in LaRacer seems very hard until you realise you can just drive outside Edinburgh and hoon around to easily build a skill chain and get three stars.
There are quite a few like this bringing you closer to 100 stars. Once you have those out of the way you should be close and feeling more confident. Some of the others are very hard in winter but much easier in other seasons, the Vulcan one and the Senna one particularly but really not that bad in the other seasons.
I played it this way and picked my chapters to get me over the line. I’m not a great racer but it can be done fairly easily never the less if you break it down.
Some chapters benefit from traction control etc turned on, others you will definitely need it off.
Final tip, Youtube has pretty much every chapter and all the tips if you really get stuck.
Hope this helps 🙂