Well, if you select your Null object and see the motion on the top view you’ll be able to fine fix your motion keyframes. Right now they are positioned in a funny way. Try to place them in the same place them as if you were drawing in Illustrator. Let the handlers follow the same direction of the straight line and adjust the curvature using the lenght of the handlers, not their angle. I home I’m explaining me.
For the timing problem, select your first and last keyframes in the timeline, then right click and select “rove across time”.
I hope this helps