The FZS600 clutch shouldn't be particularly heavy even with the stronger clutch springs. If not already done so you could try rerouteing the cable to give gentler sweeps in the bends and not clipping it tightly in position. Lubricating the cable may help. They can wear over time and cause increased friction, so a new cable may be in order.
If that doesn't solve it a more difficult option would be to fit a handlebar lever setup with a smaller distance between the lever pivot and the cable end. The down side to that is that unless you have more reach on the lever you get less movement at the clutch end with the possibility of clutch drag.
For a heavy clutch try a Triumph Trident from the 70's, that really is a feat of strength
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