If it is the distance of the throw and not a difficulty pushing the lever, what you have is what you have.
Cleaning might make it easier to push, but all the shifter does is pull cable. The pawl catches and everything stays there until you push it again or pull the trigger to release the pawl.
You can dink around with stops or cable adjustment all you want, but that lever will need to swing a certain amount to engage the pawl.
John
Edit added: In theory, you could reduce the lever length and the distance would be less, but the amount of thumb pressure would increase due to less leverage… literally.