Hey. If I understood correctly your looking for linear curve to your animation. This How do I make animations a steady speed? - Blender Stack Exchange pretty much covers how to do it. By default blender applies smoothing curve to your animation and that results to your animation start and end smoothed. When you change to linear curve your animation will not have any smoothings and should go trough with constant speed.
Unfortunately I don’t know how you limit the frames when exporting, have not worked enough with the subject. Only thing I can think of is to set the start and end frames to desired but no idea if that works.