I can assure you, it is all doable, remember I had to test it with a standard vehicle in the creator to be able to publish it.
This was what got me to go back. I started with a stock Sandking, and managed to get to the other side of the river. Once I was there, I scrabbled at the uphill to checkpoint 8 for a while, then decided to quit out and finally get myself a Sandking to upgrade.
I went back in with the fully upgraded Sandking, and after passing checkpoint 8, the rest of the race was both fun and not frustrating. I really like the downhills, particularly the first one.
Having beat it with my new fully upgraded Sandking, I had to go back and do it with a stock Sandking. Definitely turns up the challenge on the uphill climbs.
I had found a slightly different approach to the cliff edge that makes the river jump easier, so I brought my fully upgraded MW Mesa in for a try. The hardest thing about using the MW Mesa on this course is that checkpoint 6 spawns the Mesa over the cliff edge almost too far to recover. I eventually nailed the jump, and found that the fully upgraded MW Mesa took the rest of the course better than the fully upgraded Sandking did. I hit checkpoint 8 on the second try. I would say it performed best out of the 4x4s I took through.
Next I tried my fully upgraded clean Rebel. The jump was tougher, but I still got across. The climb to 8 was also tough but by then I'd learned a good angle for it and it didn't give me too much trouble. After that it performed between the Sandking and Mesa for the rest of the course. That Rebel can really climb if you're gentle on the throttle.
Having satisfied myself by completing the course with every reasonable vehicle, I went at it next with my fully upgraded Sanchez. I eventually nailed the river jump, but it wasn't easy! Every time I plunged into the river I got a nice view of the pile of dead dirtbikes I was accumulating on the bottom. After finally landing it, the Sanchez tore up the rest of the course no problem.
Definitely a fun race, but I'm not a fan of the river jump to start it off. Doesn't seem to me like it fits with the rest of the course. There's a downslope just to the east that can be used to reach checkpoint 7, by leaving the road just past checkpoint 11. It's more consistent with the rest of the course, and the challenge is actually not entering the water. I think it would be a stronger start, but that's just my constructive criticism.
I blame you and XJpostman for my garage now containing 4 offroad vehicles. ;-)