You're going to have to do a little more than just give it some 6.0 pH'd water a few times... You're growing in 5 gallon pots so take 15 gallons of pH'd water and run that amount through each pot - in the last gallon for each, add in 1/2 strength nutes.... then let them dry out before watering again. You should also check the pH of the runoff and if more than 15 gallons is required to get the pH down, run more... in short, you do this until that pH is under control and where you want it.
As far as your ppm is concerned, that's really terribly high for the age of this plant... ESPECIALLY if these are autoflowers! At their age, the ppm shouldn't be more than 600 - and that's about the max autoflowers will be able to tolerate. They're QUITE finicky! ... and with them (they're all I grow), less is more... So, if you're growing in amended soil, you're going to have to allow for that and keep what you add in terms of nutes way down... it's good that you're checking the runoff for the ppm as well as the pH... just keep doing that. I might even suggest that when you flush (above), you also measure the ppm of the runoff and only if it's well below 600 (like 450 - 500) add the nutes in that final gallon... if it's around 600 give or take 50, then add 1/2 strength nutes...
Good luck!