If I’m doing an animation with line art I usually do 1 frame form the sketch and the add a page and do line art on that- this probably doesn’t help but whatever

And I ALWAYS do line art first before any frame gets color

No i mean like say of you were animating an eye closing You would start off with the eye open And then the Final frame would be the eye closed After you make those two frames you can start animating the transition between them