Definitely add more 20 Mule Team Borax to get the pH up to at least 7.0 though preferably to 7.2 to 7.5.

Are you starting to get FC readings yet? 0.5 isn't considered a solid reading. If you add chlorine and still aren't getting FC, then I suspect you may have had bacteria convert CYA into ammonia over the winter. Given the low pH, I suspect you were using Trichlor pucks and probably had a CYA level when you closed the pool last year.

If you want to estimate how much chlorine it will take to clear the possible ammonia, you can do a bucket test using 2 gallons of pool water and add chlorine where 1/4 teaspoon of 6% bleach is 10 ppm (1 teaspoon is 40 ppm) until you measure FC that doesn't drop significantly over an hour. You could also get an inexpensive ammonia test kit from a fish/pet/aquarium store though the bucket test is more definitive since there can be more for chlorine to consume than just ammonia.