Are you sure the cable is fine, I replaced mine recently, it felt fine when free but under load it felt like a very heavy clutch.
The difference with a new cable was amazing.
You could even have clutch slip if cable is bad enough, I would be inclined to change the cable first just to be sure.
Checking out my cable afterward changing it, the in side of the sheath had raveled just above the centre cable adjuster.
Technically there is no real reason for the clutch to become very heavy after such low mileage.
Clutch is reasonably straight forward, but it does have a number of different friction plates and steel plates, ensure they go in the correct position.order.
Use a torque wrench to tighten the clutch spring bolts (8Nm) as they can easily strip the threads in the clutch boss if over tightened.