spring preload & tuning
Increasing the preload on suspension springs is amongst the worst cure for suspension problems:
You risk to permanently deform the springs by putting them under too much pressure. (They get compressed to less of the "allowed minimal length" when the fork is close to bottom out)
Your spring itself does not get harder. The spring rate is constant no matter how miuch load you put on. The preload only reduces negative travel and increases positive travel. The fork tends to "top out" on rough roads.
Not sure how pvc and fork oil go together. Fork oil is by nature rather aggressive. If you do changes with spring preload rather get a steel or aluminium tube machined.
The amount of preload you gave to your fork to sort out the problem (50mm) is a lot and points at some other cause. Measure the uncompressed length of your fork springs and compare with the length given in the manual. Fork springs sometimes start to sag when they get old and this gets worse if you preload the springs.
The 50mm preload should not cost you any travel as the spring will still be above block length when the fork is fully compressed.
As a sensible way to sort out your fork I would recommend to proceed as follows:
Check free length of springs
replace oil with standard weight as recommended by Kawasaki (oil might be too old, too little, too light.
increase oil level carefully. Never more than 5mm in one step. +10mm for 1st step is ok if the fork feels very soft. A max of +25 mm should solve the problem. A slightly increased fork oil level is a very good idea on many bikes if heavily loaded as it increases the progression of the fork spring rate.
Don't increase by more than 25mm as the higher air pressures in the compressing fork at some stage will blow the oil seals.
If the fork is still too soft measure the ratio between positive and negative travel with the bike normally loaded. You should have approximately 30% of negative travel with a normal load (rider + some luggage). Heavily loaded the suspensions should have not more than 50% negative travel.
If this measurement reveals that the fork springs are too soft find a manufacturer to custom make stronger ones. A spring manufacturer will also be able to calculate an allowable amount of preload you can put onto your original springs based on the value of fork travel and original preload plus the geometrical dimensions of the spring (d, D, n, l). This is easy for a linear spring, a bit tricky for a twin rate or progressive ratio spring.
Blindly I would not recommend to put more than 5% of its free length of additional pre load onto the spring.
Do not use heavier fork oil as this will make the fork feel hard and ill responding and will cost precision on bad roads.
Maybe the best solution will be slightly preloaded fork with slightly more oil.