That's an awful lot of borax to have no effect. I'm wondering if you might actually be super-alkaline and that's why.

But rather than waste a gazillion boxes of Borax, why not take a five gallon bucket of pool water and add Borax to THAT until you get a reading? I'm not sure how many cups of Borax are in a box, but you could start with 1/8 cup and see what that does, and add 1/8 cup until it registers. That will give you a baseline. If it STILL cannot do it, try going the other way.

Take a 5 gallon bucket of pool water and add about 10 ml (an eyedropper full) and see if THAT now registers--adding acid until it does.

If your pool really is super-acidic then the added bonus of adding all this Borax is your borate level should be approaching the ideal of 50ppm--which helps inhibit algae and helps keep pH stable.

I dunno--that's how I'd approach it.