I've been playing a lot with Gr.3 races, mainly because well, I like my Vette and because I'm testing the oil/engine wear.
If, for whatever reason, you don't want to race in rally or Gr.4 but you still want some credits, then Gr.3 got you covered!
Again, nothing "meta". This is purely because of fun, because you can't stand the other events or because you can't stop thinking about that lovely Gr.3 engine noise.
Anyway. There's four Gr.3 races with 5 laps, all* of them paying 50k + 25k if you get the clean race bonus.
*La Sarthe is also 5 laps, but 5 laps there equals to more than 18 minutes. It pays like one of the 10 laps race (70k)
Spa-Francorchamps - You can beat it in total time of 12 minutes. Although it's "slow", it's by far the easiest regardless the difficulty you're playing because the AI really struggles with Eau Rouge. The track is wide and overall it's just a cozy 5 laps with an easy clean race bonus.
Lago Maggiore - it's the fatest circuit with a total time of 10:20 minutes. Just be careful with the AI because this one can give you a tiny bit of trouble (or I'm just bad at this track). At the easiest difficulty the AI finished less than 2 seconds behind me and at the hardest difficulty the AI actually came quite close between 0.5-1s! (now that I'm thinking about, I'm really just bad...). Also very easy to get clean race bonus.
Suzuka - total time of 10:30 minutes. Between this and Maggiore, well, pick your poison. The problem with Suzuka is that I find it easy to lose the clean race bonus. The track is tighter and it's easy to collide with the AI, or better saying, the AI colliding with you!
If you're thinking "what about doing one 10-lap race versus doing two 5-lap races?". Those 10-lap races are paying 70k, so that's 105k with clean race bonus
All 10-lap races have tire wear and fuel consumption so you have to pit. Catalunya, Maggiore, Monza and Dragon Trail are 20 minutes minimum. Fiji it's a rainy race, so it's even slower. La Sarthe may be a good alternative, since you can finish in ~18-19 minutes, but that's still 45k credits below 5-lap races, just like Daytona where you can finish in 18 minutes.
TL;DR: basically in ~24 minutes you can do two 5-lap races and earn 150k or one 10-lap race to get 105k.
If you ask me, altough those 10-laps races are really fun (especially Fiji!), right now I prefer the 5-lap races. I think they're fun enough for an hour or so of grinding.