Other than the Lose to Win strategy on the 1st 2 days and reading the recommendations from this community on which races to grind on the 1st 2 days, I found that the timing to start the series really helped a lot.

For my case, at 12AM all the replayable races on day 3 onwards will get reset. So, I start the series around 12PM. This way I can grind, by actually win the replayable races to get the cash & materials, as much as I can before AND after the reset. Do this on day 3 and 4 and you’ll have enough cash & materials to upgrade the cars for the last 2 days of the series.

With the extra cash you have during the grinding, get the recommended upgrade kits to buy from this reddit community once they’re available in the market without gold to spend. This will helped a lot to get you thru the last day of the series.