I'm only on race 6 of my first championship and I have earnt over 500k already with about 3 hours ingame time on stats, I don't think the grind is as bad if you're playing it legit. The game really isn't that difficult if you take some time to learn each track. I'm currently racing against hard AI on medium length races and winning by over 10-20 seconds, considering bumping the AI up to realistic for more credits but that might be too unforgiving.
The biggest tip I can give you is turn on qualifying sessions and follow the AIs lines to actually learn each track and the flow for the jumps, I feel like many people are treating this like a car racing game when its completely different. If you are struggling to get enough height for some big jumps, go wide on the berm of the corner before the jump to get a little extra speed.
You need to learn the correct flow for each set of jumps, as when you get it wrong it is MUCH slower. Cornering is completely different to cars. You often need to straightline into corners and do a quick 180 turn and accelerate again the other way (watch what the AI do in tight hairpins to see what I mean).
Turning on manual weight transfer is actually much faster than leaving it on weight transfer assist, as you can use it to lean back before big jumps to get extra speed as well as clear some jumps a little easier by moving your weight forward in the air before landing.
As I said above I'm currently 6 races into the first championship and beating Hard level AI pretty handily with a fully upgraded 250 bike, this game is actually a lot of fun once you get it right and I'm pretty disappointed with all the people dismissing it as a terrible game within an hour because they can't be good at it instantly.