To be rather honest I find FF soil to be weak in the nutrient department, I mainly just use it because its light and fluffy. Also remember depending on your strain it could easily use up the available nutrients.
Don't buy Grow, get the fish mix. Also your chart is for soil, FF is predominantly coco.
I feed with every watering. Feeding at your current level of 6 is fine, the issue you have is the lack of the nutrients available.
I'm usually feeding around 5.8-5.9. Feed with calmag, then after this watering start your nutrient feeding at half dose for a few feeds, monitor the plants then increase to full strength.
I went the same route you did was in FF seedling mix, it started showing those signs then moved to the soil and on the initial feed I helped it along with some calmag.
It's calmag I had the exact problem. Feed calmag and it should bounce back. Also feeding every 3-4 days with no nutrients you're basically just flushing out everything else. Start your feeding cycle with some fishmix.
It was not a double tune, I merely stated its dangerous, and it can lead to death. I have seen first hand what the miss handling of solvents have done to people with far more training than anyone here, proper chemical engineers/chemists back in my day at uni (biochem department) , and yes I offered to source the products for him because he's an adult and I'm sure he will use common sense and as he stated he is aware of the risks.