Morning,
So at first glance my instant thought was light burn, as I had a plant display a similar white/yellow spotting. If your ph is 6.5 there's nothing wrong with that for soil growing, it can go up to 6.8 and still be safe. You dont seem to be giving a surplus of nutrients to burn the plant with. But you are using HID which is very powerful and hot. All your stats look good, my one possible suggestion is raise the light to 24inches and lower your hours to 18.
Also plants thrive on having a wet period and a dry period, the dry encourages the plant and its roots to seek out water, aka more growth longer roots etc. I wouldn't water excessively if I were you, it wont solve any ph problem that I know of.
Best of luck, plants are truly resilient, I dont doubt she'll be ok, just needs some adjustments. ✌️🌱