Hi, thanks for the question. Sorry for the long story but this might help some people.
I've done a LOT of grinding and completing a LOT from general gameplay.
I've completed every single race, mission challenge, endurance race, circuit experience (including full laps) to gold which gave me tens of millions of credits (Nurburgring full lap gold is 5,000,000cr alone). There's literally no race left for me to complete to gold.
When the Dodge Tomahawk exploit was available, I did that Tokyo race many, many times - probably at least 100-150x I would say. When I couldn't do it (like now), I've been racing the Sardegna 700 World Touring race which gets me 727,000cr every 25 minutes or, the Le Mans 30 min 600 World Touring race which rewards you with 825,000cr.
I've mainly played GT7 pretty much since it's release in March with the exception of a month in the middle of the year when I played Tiny Tina's Wonderlands.
It's been a HUGE grind but all I do is 3-4 races per day which gets me 2.1-2.8m credits per day. I try to approach it with a mindset of every day and race is another step closer. At the moment I've got 38m credits but the 4 remaining cars that I need are: Alfa Romeo8C 2900B Lungo Berlinetta 1937 – 20m crFerrari250 GTO CN.3729GT 1962 – 20m crMcLarenMcLaren F1 GTR Race Car 1997 - 9.5m crMercedes300SL W194 1952 – 11m cr (this is supposedly 20m credits now but I'm unsure!)
Lastly, when I set about trying to collect all the cars, I kept my objectives simple and manageable. For instance, I set about buying all the cheap cars in Brand Central and all the used cars. Then I focused on all cars below 500,000cr and then all 1,000,000cr or less....then anything under 5,000,000 etc...sometimes I'd buy cars I really wanted or the cheaper legendary cars. When I got to 300 cars, I knew I could get 350....then I just tried to get 360, 370 etc...cars. When I hit 400 cars, I knew I could do it.
It's also crazy because once they allow players to sell vehicles, I will potentially earn many millions more as I have at least 25-30 cars to sell.
Once I'm done with all the cars, I will buy all the racing paint jobs - I have all the general ones and all the racing wheels/mags. My goal is FULL 100% completion. I have the platinum trophy too which I got months ago.