I am first time player of Forza and love it. Want to share how I attacked the game.
I went league by league and completed each one 1st. I usually would go for the recommended cars, and if I already owned a car for that one, I didn’t buy any. This only backfired once in the track toys. I went for the open cockpit roadster lotus and noticed the other lotus available kept kicking my keister.
When I would run out of money, I would go back and go for gold in the unlocked tracks in prior leagues. This only happened to me when I was on the final 2 leagues. I didn’t have 1.6 mil for the 1970s indie and modern indie 1.3 mil credits.
After completing the leagues,I went for gold on all the ones I missed and the unlocked tracks.
Yes, there is a credit grind method, but you don’t unlock as many achievements that way.
Hope this helps. Keep racing and smiling!