Can a single line of Gcode have variable extrusion rate?
My understanding is that the printer firmware will define the maximum acceleration and speeds for each axis (X, Y, Z, and E). When executing a line of g-code that involves more than one axis, the acceleration for each will be limited such that they all begin and end, including acceleration together.

During the start and end of a line, when the print nozzle is moving more slowly, the extruder will also move slowly. In the middle of the line, the extruder will move faster. The amount of material extruded per distance will be constant over the length of the line.

Another way to look at it - suppose you could accelerate quickly on X with a high top speed, but slowly on Y. A move only in the X direction could happen quickly. A move only in the Y direction would be slower. If you want to move at a 45-degree angle,  you will need to slow down X so that it doesn't get ahead of Y.

If you did want to intentionally vary the amount of extrusion per distance during a line, you would need to break it into multiple segments with individual lines of g-code.