I took a look into challenge mode and came up with some strategies for 100% if someone wants to speedrun it. Including some smaller breaks, it should be definitely possible to finish it in less than 11 hours. Maybe someone wants to give it a try in the future.

Two strategies to mention beforehand. The first one is: Always put on a custom setup, even if it takes some time. A good setup can give you up to 11 seconds of free lap time every lap. I recommend the following setup (but of course you can build from there):
Brakes in the centre, maximum wings, minimum tyre pressure, minimum ride height, maximum anti-roll bar, front suspension individually for each car, but usually between 6/12 and 9/12, maximum rear suspension, stock camber, stock toe.
Most cars have a good gear ratio, for some you might want to make the gear ratio shorter or longer by up to 3 clicks.
The ballast position is the only thing that varies greatly between each car. It is just a matter of preference though.
The second game mechanic you will have to consider is the way the game handles ties in challenge mode. If two drivers have the same number of points in the end the game will favour the driver who has won the last race of the challenge. I will mention it in the challenges where you have to look out for it.

Also, before starting the run, apply your custom setups to all unlocked cars in time trial. These setups will then carry over to challenge mode, as long as you do not quit the game.

Now let’s go to each challenge individually:
Abt Audi TT-R: Skip Hockenheim. If the leading AI opponent has 17 points after 2 races, skip qualifying of the last race in order to find out which position they qualified in. When passing them at the start of the race, ram them off the road. In manufacturer races the AI usually won’t be able to recover all positions after the crash. This quick ramming strategy is viable for all manufacturer challenges that have 3 races (though not always necessary). I won’t mention this strategy for any following manufacturer challenges.
GT Cup: Use the Noble. Skip Silverstone. In Monza you will have to wait for the AI on the last lap in order to ram the BMWs off the road. I recommend taking them out at Ascari (the fast chicane at the end of sector 2). You will have enough time to make up any positions lost, but the BMWs will likely finish in P7 and P8. Skip Hockenheim. In Springfield Short you will have to wait for the AI and take out the BMWs again. I would do it in the fast left hander and the following fast right hander in sector 2.
Noble: Skip Hockenheim.
British v German: Use the Noble. Skip Hockenheim. On the Silverstone race, ram the BMW off the road when you pass them at the start off the race. For some reason the BMW slows down on the final lap of the race, giving the Quaife enough of a lead to keep it to the finish. This strategy is more consistent than ramming the BMW at the start of the Motodrom race.
Quaife: Skip Monza.
Euro Cup: Use the Quaife. There are 2 possible strategies. The safe strategy is to only skip Hockenheim. But you can get away with skipping 2 races. You will have to look out for the way the game handles ties as I mentioned before. Since you want to skip Hockenheim, this would put you at a disadvantage in case of a tie. But you can do the following: Skip Monza. If the winning BMW has the fastest lap, restart the challenge. If someone else has the fastest lap you can go on to ram the BMW at the end of race 2 and 3 so they finish in P7 and P8 just like in GT Cup. Then skip Hockenheim.
BMW M3: Skip Hockenheim. (No one likes Hockenheim) Also, make sure that whoever finished in P2 in the first race does not finish in P2 in the second race.
German GT: Use the BMW. No strategies. Just go as fast as you can.
Sprint Cup: Use the Quaife. It has more grip than the BMW and roughly the same top speed. Skip Silverstone International.
Sintura: Skip Silverstone International.
GTS Cup: Skip Hockenheim and Monza. You will also have to wait and ram the McLarens at the end of the races you do. Though, depending on the championship standings you might not have to ram anyone in the last race and can just drive off into the distance.
Panoz Esperante: Skip Monza. Also make sure that whoever finished in P2 in the first race does not finish in P2 in the second race.
Storm Cup: You can choose either the Sintura or the Panoz, whichever you like more. Skip Hockenheim. Also, you might need to wait for the McLaren and ram him in one of the other 2 races.
Vemac: Skip Hockenheim.
Sebring Cup: You can choose either the Panoz or the Vemac, since they have roughly equal performance. No strategies.
Lister Storm: Guess what? Skip Hockenheim.
Trans Atlantic: This is the biggest skip in the game. You can skip the entire 15 lap Silverstone race and finish level on points with the leading AI driver. You cannot skip Sebring due to the game favouring the driver who wins the last race when tied. The strategy works as follows: Use the Lister. Skip Silverstone. In Sebring you ram the other Lister when passing him at the start. With the setup I wrote at the beginning you are fast enough to lap the AI 2 times if you don’t make too many mistakes. When lapping the other Lister for the second time you will have to ram and block him until the slower Panoz overtook him. Then race to the finish.
McLaren F1: Skip Hockenheim.
British Cup: You can use either the Lister or the McLaren, though I think the Lister is both slightly faster and more consistent than the McLaren. Choosing the Lister makes it also easier to ensure the AI McLaren who wins the skipped race does not finish right behind you every time. Skip Silverstone International.
Far Eastern: Choose the Lister or the McLaren. I would choose the Lister as mentioned in British Cup. No strategies here.
BMW V12: Skip Hockenheim. Also make sure that whoever finished in P2 in the first race does not finish in P2 in the second race.
Prototype Cup: Skip Monza, Sebring and Hockenheim. No need to ram anyone here since the AI Audis, who will win the skipped races are absolutely atrocious when actually racing and will usually finish outside the points on its own.
Dome: Skip Minato City, since 4 laps of Minato City take longer than 3 laps of Hockenheim (good for you Hockenheim, that’s the first time we don’t skip you when given the choice)
High Speed: Choose the Dome. Aaand… we’re back to skipping Hockenheim again.
Panoz LMP: Skip Hockenheim.
Fast and Furious: Use the Panoz or the Dome. They are pretty much equally matched. Skip Hockenheim, Minato City and Sebring. Also, if you are really lucky and the Audis both take away points from each other in the skipped races and perform badly in the other races, you might be able to skip the last race in Rockingham. It’s only one lap though, so not a big time save.
Bentley: Skip Hockenheim.
Traffic: Use the Bentley. Unfortunately, you can only skip one of 6 races, since you only have one opponent. Skip Sebring.
Audi R8: Skip Monza, because 4 laps of Monza take longer than 3 laps of Hockenheim. (Wow, another Hockenheim race that is not skipped)
Audi v Bentley: Use the Audi. No skips possible.
Pilbeam: Skip Monza and Silverstone International. You might also need to ram off the AI driver who wins the skipped races in all remaining races when passing them at the start of the race.
Sebring 50: Use the Audi. No strategies here.
Lister LMP: Skip Monza.
Bonus Car: Use the Lister. No strategies here.
Intercontinental: This is an interesting one. Choose the Panoz or the Dome. Do NOT choose the Lister, even though it is a lot faster. The reason for this is because you want to have 2 Lister AI opponents. With the setup above you can still quite easily beat the Lister AI. You do the following: Skip Sebring. At the beginning of Minato City, you ram whichever Lister won the first race, so that he is behind the other AI Lister. He will most likely not be able to reovertake the other Lister, but he would easily overtake the Panoz and Domes, because the Lister is so OP. Proceed to finish Minato City and Monza as quickly as possible.
Rockingham Oval: Use the Lister. No strategies here.
Rockingham: Skip Rockingham.

This looks like a lot, but it is mostly about remembering which circuit is the longest and which car is the best and only a few special strategies.

I did some more testing and found out that you can make the cars another 1-3 seconds per lap faster by lowering the front suspension only. You cannot lower it all the way or else you will only spin out. The sweet spot is somewhere between 6/12 and 9/12 for most cars and can vary between each car just like the ballast position. I updated the information in my main post in order to avoid confusion.
I also retested the setup with different camber and toe values, but they never gave me any extra pace, they only gave me a different feeling during cornering. So I still recommend keeping them stock.

I found a more consistent strategy for British v German: Instead of ramming the AI BMW at the start of Motodrom, do it at the start of Silverstone, because for some reason the BMW slows down on the 3rd and final lap. This is giving the AI Quaife enough of a lead to stay ahead when the race is simulated after you finished the race yourself. I will once again update this information in my main post.

I will also add the information about applying the car setups for all unlocked cars before starting the run.