The more you pay, the more they'll apply to principal. E.g, when you doubled up on the 19th, they applied all of the 2nd payment to principal. So just pay as much as possible as soon as possible, and that will be the way to accelerate the paying down of the principal (and reduce the amount of interest).
Making extra payments when you can, will save a lot
more than just switching to an early payment date.
By moving your payment date 3 week early,
at most you will save $50 that first month.
Amorizatied over the loan gain another $40.
So $90 from switching payment date.
If early in loan an extra $100 will save the $100
plus another $50 over loan term.
Extra payments are the real key if wanting to save money
and also shorten the loan time.
Pay as much as possible as soon as possible.
Early in the loan the gain from extra payments is more important