Do you feel like you're chasing your own tail yet? The last few times I used to tupur, it came pretty high PPM out of the bag. I was facing too many drying out issues as well. So I switch to a low EC, Pure coco mix. Recent Brands I've been using our char coir, and Royal something or rather, the most important thing is well flushed low starting EC, and pure coir.
I'm also using Jacks between 1 and 1.4 EC, and 5.8 to 6.2 pH. I'd say your problem was definitely drying out between waterings. Ec spike in your medium in PH swing wildly. I'd recommend flushing with v e c of Base nutrients till you reach under 1.5 EC, then feed with one EC while checking run off every week to not exceed 1.8eC. If you get to 1.8 PC or above, then flushed back down to 1.5 EC again. Then figure out how many waterings a day you need to keep the medium consistently wet so the EC does not Spike each week. I'm in 5-gallon pots, and in flowering I'm feeding around 6 to 10 times a day for an average of about 2.5 to 3.25 gallons per day.