The spring that you are working against is the one(s) that clamps the clutch plates together. These have to be strong enough to prevent the clutch from slipping, so you can't do anything about that. (Aftermarket ones are often stiffer to reduce the potential for clutch slipping.) So potentially you could increase the leverage at the bars. You can't reduce the movement at the master-cylinder/cable side of the lever as then the clutch wouldn't fully disengage, so all you are left with is extending the hand part of the lever, and you can buy those. But, imagine that your existing lever continues to the left, it is also going to be further away from the bars, and if you have hand-guards, probably hitting them.
Or, (and I'm not joking) you could strengthen your left hand, like I have been advised to do because of arthritis, using bits like these:
https://www.ebay.co.uk/itm/335450000831