Late to the party, but I’m playing through Horizon 4’s story for the second time and here’s what I’ve got:

• If you rely on the driving line, don’t. 9/10 times they’ve tuned these things so that you have to use shortcuts of one description or another to hit 3 stars. Especially those roundabouts and exits - speed through them and don’t slow down just because the driving line says so.
• Use rewind. It’s so much easier to practice and refine difficult portions of a challenge by simply rewinding and doing a shitty portion better. Some challenges are 3-5 minutes long (or longer) and will demand practice for 3 stars. Rewind doesn’t cost you time either so lean on it for those tough bits.
• if you’re seriously struggling with a particular challenge, take a break and come back to it another day. You would be surprised how often this works.
• Final tip: lower your standards a little. I believe the achievement on consoles is for 100 stars, not 120? Regardless, you don’t need 3 stars on every challenge to mop up most / all progression rewards. I think there’s maybe one car per story/business locked behind a perfect rating; if you can be okay getting almost everything, 100 stars is more than enough.