The problem with thinking about the senario from an initial setting only.

Let's say that initially then motor is stopped and the joystick is at center.

Next you enter into speed control mode.

Move the joystick and motor speed follows the joystick position.

Release the mode control button and memorize the the memory speed.

Finally release the joystick to pop back to center position.

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Now some time later you come to operate the joystick again.
What do you do.....what position of the joystick corresponds to what motor speed.
Any solution you think of will likely not be a very good one...

...unless you have the MCU simply detect the joystick resistances at above or below some set value thus turning the joystick into an effective equivalent of switch contacts that act as + and - controls that bump the speed up and down.

Michael Karas