Everything you do is rightly done. Now you only can hope that spawning will take place.
P. parvulus often likes to spawn unser a leaf. Often in this case one is unable to locate eggs because of the mass of the leaves. And if they don’t spawn under leaves but do it in a firm cave, then – possibly – they transfer the eggs under a leaf, later. Or from one leaf to another. The parvulus like especially very small caves only with half an inch or less in diameter.
I hope that this time you placed the cave in such a way that you could easily inspect it with a torch. If not, you surely could correct it now with the male in it. But correct it in slight steps only. One a day.
Reducing the pH to 3.5 is a right action, too. Now the danger of an aggression to eggs by germs is much reduced. Nevertheless you must control the eggs (if there are some). They must stick under the ceiling of the cave, not lie down on its bottom. The last will inevitably result in egg-loss. But the first might also, by unknown reasons. But all you can do against it has been done.
If the eggs lie on the bottom of the cave, this is most likely caused by water too hard. But I don’t think that this will be the case in your tank; otherwise you could not reduce the pH to such a value.
If they spawn and there are eggs on the ceiling cared for by the male, then you must control whether larvae hatch. At first the larvae look like longish eggs. But some days later they become more longish and blackish, and the they sometimes move, but still fixed at the cave’s ceiling. The male is the best guard for them. Taking them out in this stage mostly results in loss. You cannot substitute the intense care of a caring male.
There are three methods to proceed after about a week. First, leave everything as it is, the adult fish included. I should recommend that if there is much of hiding place on the bottom of your tank. The pair will spawn several times. Second, you could take the adukt pair out, but only after the young have left the cave. This would increase the surviving prospects of the young. But normally adult parvulus tolerate young at their side. Third, you could pipe out the young that dash around in the cave into a small jar with exactly the same water and frequnet water change. But then the controlled feeding is most important: not too little, not too much. Perhaps you should do so with a later spawning in order to have a second chance.
At any rate you need patience now. It sometimes needs four weeks or more (!) before you see any young. Good luck!