When it comes to grinding for Cr. in GT7, you're repeating the race/s over and over to acquire the credits, regardless what race you do it in, repetition=results. In terms of the quickest time and most payout per race, Tokyo still takes the cake even after all these updates. If you know of a better method, by all means share it with us.
Strategy:
Car: V6 Escudo Pikes Peak on comfort hard(CH) tyres for the fronts and comfort medium(CM) for the rears, High-RPM Turbocharger.
This setup with downforce and gearing settings(pics below) gives the car a 1,152 BHP and easily sneaks in under the 600PP regulations for this race at 583PP. I always lap all the AI cars twice with this setup. You can experiment with the gearing to shave a few more seconds here and there if you have the time and patience.
Wall ride everywhere except at the last turn(hairpin). When wall riding, I find that pushing the left analog stick away from the wall achieves a faster consistent time with each lap. Be careful not to touch the hairpin lines at the last turn or bump into the AI throughout the race, which isn't hard to do once you get the hang of it and you'll get Cr. 825,000(with CRB) in about 21-22mins. One of my fastest runs was achieved in under 20 mins(tinker with gearing), but that required full concentration, whereas the 21-22 minute runs are much more chill and easily achievable. I have no probs getting the CRB every time I run it.