pretty much all the beginner tracks, oil crisis, rock steady, roller coaster, physics factory...i did kinda name all the ones i have plat. but thats cuz i thought they were easy to plat imo...maybe some others but havnt gone thru trying to plat levels quite yet....most of the skill games are pretty easy to plat as well imo
Medium/Hard tracks should definitely feel doable if you have already golded the Extreme tracks. For me, some of the Beginner tracks took me several hours to platinum, you really have to hit a perfect run on them with forward flips, landing on the back tire etc, and I always watched a few of the videos on the 500 - 1000 leaderboard rank for any bits that could be done a certain way to shave valuable seconds off.
As for the skill games, I've got Plat on all but 2, and those 2 are giving me nightmares. The Ball of Steel one, balancing the ball on the bike attachement, that's nasty without using the glitch. But the worst for me is the "full throttle" one where you have no brakes and auto accelerate, I've got to the same point over and over and still can't get to 300m, it's the only thing I've been doing on Trials lately, trying for hours to get past it.