The first 2 seasons i did ( although in rally online career ) were easy. At Pro i can not win every rally anymore, but i still win the season. Haven't gotten into my 4th season yet. Same thing should apply for RX.
For the weather, one of the last updates saw a decrease in rain %. So New championships should have less rain. Don't think existing ones change.
If you go to freeplay. The 2 left buttons are oldskool offline career modes. You just join a racing team and race.
In historic, you start in one of the old rally cars racing teams, and you go through 4 decades. But here you can choose your difficulty beforehand, and "hardcore" damage. I guess the fia wrx button is kinda the same thing. But i guess just going through a couple of classes.
This career mode doesn't count towards cash and car gathering. It's just like the F1 games where you just join a team and go. And build yourself up into the fastest teams/cars.
