I brought cars cheaply then sold them on for profit, as well as goliath race in a HE car. Although I no longer play now so not sure if that even works anymore
Create a new blueprint for a bucket list challenge, make it 30 min long, and drift your ass off in a HE car with skills boost, I do that for 15 min and get over 300k xp, 500k credits, not too much but it's easier than Goliath
Just playing i mean... when you just love playing and not obsessed with all these "i wanna be a billionaire and have more cars then others" stuff you just go and play, and you know... after 3-4 months you look up in a right top corner and see 8 000 000 CR...