Update Nov 2021: This farm still works. The Rally Trophy track breaks the economy in half.

The Big Picture concept: You're gonna outlast your opponents in a race designed to get them to take themselves out that pays out faster than you really should.
Payout: (on average) 190K XP 48K credits per race.

The New, But Less Fun Method:
Step 1. Choose Custom Event. Create a 60 lap banger race on Rally Trophy. Set AI difficulty to Expert. Vehicle damage to Realistic. Driving difficulty can be whatever you want. It doesn't affect cash payout but does affect XP multiplier so use what you're comfortable with/find fun.
Don't worry about the 60 laps. You're not going to drive a complete lap of that.

Step 2. Choose any car you like. Ideally one with as much armour/strength as possible. Your goal is to outlast your opponents so get a car that can take a few hits but you'll need enough speed to keep up with the leaders. AI vehicles should be set to something that's as weak as possible like the sofa car.

Step 3. Your goal in the race is to wreck everyone else, or cause them to wreck themselves. This shouldn't be too hard as the trees, crests and embankments are great things to push other drivers into. Try to preserve your car as much as you can. Ideally get to the lead then start wrecking. This process will take time but the payout is worth it, even if you wreck yourself trying.

Step 4. When all other racers are wrecked (you'll know because all the drivers aside from you are listed as DNF in the top left), wreck yourself and it'll count as you winning despite not completing anywhere near the 60 laps.

Step 5. Rinse and repeat, have fun. Don't worry if you wreck too early/don't win. You'll still get a considerable payout, which is more than most events give in my experience, so you're still making bank.
TL;DR - Rally Trophy breaks the economy in half. Wouldn't be surprised if farming servers pop up and it gets nerfed.


The Old, Less Effective, But Fun Method:
Step 1. Choose Custom Event. Create a 60 lap banger race on Hellride. Set AI difficulty to Expert. Vehicle damage to Realistic. Driving difficulty can be whatever you want. It doesn't affect cash payout but does affect XP multiplier so use what you're comfortable with/find fun.
Don't worry about the 60 laps. You're not going to drive a tenth of that.

Step 2. Choose any car you like. Ideally one with as much armour/strength as possible. Your goal is to outlast your opponents so get a car that can take a few hits.

Step 3. Race as normal but try not to take damage. Because the AI is suicidal on this track and they're basically guaranteed to take damage every time they pass the middle section of the track, whether that's hitting the wrecks of other racers or the result of going over the ramp at a wonky angle.
Arguably you don't even need to race and can just wait around watching the other cars fly over the ramp but I imagine you probably want to do something more engaging while you wait. Completing laps also increases the payout.

Step 4. When all other racers have taken themselves out (you'll know because all the drivers aside from you are listed as DNF in the top left), wreck yourself and it'll count as you winning despite not completing anywhere near the 60 laps. The AI will usually take itself out by lap 6 (or lap 3 with the right AI vehicle selection) but often it'll happen sooner especially if you carefully contribute.
I usually wreck myself by practicing spinning and barrel-rolling my car off the ramp but anything'll work because Realistic car damage makes your car as durable as tin-foil.

Step 5. Rinse and repeat, have fun. Don't worry if you wreck too early/don't win. You're practically guaranteed at least 2K in cash, which is more than most events give in my experience so you're still making bank.
Try to have a laugh with driving up the banked ramps as high as possible and laugh as the track becomes a Mad Max hellscape lined with wrecks by lap 3.


Other Information: Based on feedback by commenters and further investigation by myself, it's been found that vehicle class has no impact of XP/credit payout.
Some have suggested setting AI vehicle class as the Lawn Mower, which does make for easier AI wrecking but I find they damage my car too much (less than my previous stated A class suggestion but still).

My new recommendation is to set AI vehicle class to Sofa Car as it's less durable/lighter than the lawnmower and way less stable, especially over jumps and being rammed from in front/behind. In a good run you can actively hunt down the other drivers while racing and be done by lap 3 instead of 6.

Alternatively, if want a route that's not as fast as the Sofa Car method but faster than my original method and arguably more fun, set AI vehicle class to Super Van. They're fragile, light, unstable, easy to roll over by just pushing them and when they crash out on the jumps they tend to do spectacular cartwheels and flips.

For more fun, but a slower farm try setting the AI vehicles to motorhomes. They smash up real good and leave debris everywhere (may be an issue if you use a lower spec computer) and deform like a soda can when hit or roll over. The ice cream truck is also a fun choice but will require the player driving one and setting the AI vehicles to Same As Player.

Steam User JBatic in the comments has found that using my method on Motorcity Circuit against Lawn Mowers gets about 12K in credits. I can confirm this is indeed true however Motor City has little obstacles to cause the AI to wreck beyond ramming them yourself. I find I tend to wreck before my opponents, which still gets a handsome payout, which isn't as fun to me so I'll leave it up to you if you want efficiency over fun.

Extreme vehicle damage:
This option allows for roughly the same completion time as Realistic damage. Could be used to achieve faster results as each rival car only needs to lose a pair of wheels to be wrecked as opposed to their HP being reduced to zero, which if you're aggressive could be faster. Downside is that also applies to your car as well and every ram will damage your wheels so care and strategy is required. Overall Extreme damage should be used if you want to have a bit more fun as you farm, even if it takes longer to get rewards.