Hi! I'd recommend taking Get Ready for AP Calculus first - try the course challenge once or twice, get mastery credit for the skills you know, and identify what you don't know. If you get an 80-90% on that course, perhaps move on to either Calculus 1 or AP Calculus AB first? Then the same course challenge/unit test, then fill in with lessons method, and try advanced un-mastered units in Calc 2 or BC, then multivariable calc?

Calc 1 and AP Calc AB are equivalent courses, and if you're giving the AP exam, the AP course should work. Otherwise, either should be fine, since both are intro college-level courses.

I'm afraid the mastery learning philosophy of Khan Academy isn't about fixing the time needed to master. But, paradoxically, since the goal becomes competence over checking off boxes, one masters the material faster than expected oftentimes!