Ok a couple of fundamental misunderstandings here, but very common.
1. Increasing the preload does not make the spring rate stiffer. They are the same springs just they are compressed a bit more. It does more to change your ride height, even if it does feel stiffer when you push up & down on them). Thick wall PVC pipe works just fine as a spacer though. You need to change the spring rate to reduce the bottoming. This can be done by increasing the diameter of the spring wire (not easy) the coil size, or reducing the number of active coils. This is cheap & fairly easy but does require some thought as to whether the spring will coil bind when fully compressed. You cut off a number of coils, heat the top end & bend it flat. Then you replace the reduced length with a longer spacer. For example in my old KDX I think I cut 3 coils off 30 total (10%) (memory only) & it was good. Newer ones I am told would coilbind, you’ll have to measure spaces from a preloaded state as it would be in the fork. Same trick can be done on the shock but you’d only cut off a section of a coil.
2. Increasing oil level will not increase compression damping. Thicker oil will but also the rebound, a little thicker may be worth a try. More oil will decrease the air level & increase the pressure near the end of the stroke which might help a little up to a point.