Funnily enough I was just on a Kahn Forum post from 7 years ago noting how the Kahn community needs more development because at the moment it is difficult to find, and difficult to use and perhaps as a result there are very few reponses on it.  I came across this post as I was trying to find out how to use the Kahn Academy interface and thought I would answer as it does not seem to have received any love.

I am an adult now, I have finished high school, I have my Bachelor of Science from University and have my PhD, worked as a University lecturer for 24 years and am now working in industry.  I was always terrible at maths and would have failed very badly if it was not for one amazing teacher who came around to my house and tutored me in his own time and would never accept any money for his help.  He was the best.

Any way I have always wanted to go right back to the beginning and see if I could learn Maths partciularly with all the resources that are available to help students learn these days (which definitely weren't available when I first went through).  So that is what I am doing on here.

To answer your question, from yoru description above you are interested in getting a grounding in calculus, but not somthing that is going to be a struggle the whole way through.  I wonder whether the introductory courses:

https://www.khanacademy.org/math/get-ready-for-precalculus

https://www.khanacademy.org/math/get-ready-for-ap-calc

Would be good introductory courses to wrap your head around Calculus.  And then if you find they are pitched at a level you are ok with, get into the more difficult ones.

The first of the more difficult ones to do is:
https://www.khanacademy.org/math/ap-calculus-ab

Which is the lowest level of Highschool/University level Calculus courses which seem to have been rolled into one course (they used to be AP Calculus AB and Calculus 1 respectively on Khan Academy).

The next most difficult Calculus course is:

https://www.khanacademy.org/math/ap-calculus-bc

So maybe give those a go.  Just my thoughts.