Besides the glitchers, and there are a lot of them I will say, the best way to make cash is just to race. I have not done one of those glitches and I have been racing a lot, even had a dew days that I did nothing but drift and make tunes. And you can easily tell a glitcher by looking at how many cars they have vs driver level vs time played. For instance, I have 2 series, or 12 races total left in the entire campaign. I am driver level 183, Collecter tier 21, 247 cars, with a total time of 5D, 17H, 54m. So I do have some valid input on how to collect the cars the correct way and not bug yourself.
Oval tracks are good for races and they have some additiional perks people often overlook. For instance, it is much easier to put the difficulty to unbeatable and get that extra 100% credits at the end. As far as I know, there is no additional changes to using automatic vs manual, damage vs wear on tires, so keep things simple and just race with cosmetic dmg and no wear and tear and drive the oval. Even with a basic stock nascar, you can easily win one of these 50 lap races and get 100K+ in about 30 - 60m (depending on the track). Now if you want something harder, set that freeplay to a timed race and put it for a few hours and not laps. This will get you more credits than just a lap limit and will take longer.
Another one is to find a race series you enjoyed and change it from normal laps to the extended long race. This can drastically change the laps needed to complete the race. For instance, the very first race in the Nascar series is a 11 lap oval race. Changing it to the extended long races makes this a 100 lap race. So keep that in mind.
As far as mods, you get out what you put in. So using a mod on these small lap races are not doing you any justice than say using them on an endurance race. IE would you rather get 100% credits from a 10,000 credit race, or a 140,000 race? Using the mods strategically will extremely boost your credits and chances of being able to purchase that new shiny car.
If you want a insane car for these oval tracks, look at that renault indy car. It is stupid crazy and turns on a dime, literally. A easy way to find it is in the Forza cup, I believe the top right series. It is a `.5m credit car with maxed accel, turning, and braking.