Yellow leaves

Leaves will start to do this in flower. Especially ones down low that aren't receiving enough light. From the looks your light isn't so powerful which is evident from your stretch and node spacing. Just remove the dying leaves and keep going. Nothing to fret over. 🤘😤

Ph should be closer to the 6.5 around flowering time. Calcium and magnesium are more available at higher ph. If ph is to low it will be lockout.

Have you checked your ph going in and your ph going out (run off) also don't forget to supplement nitrogen during the frisrt week of flowerin and slowly reduce nitrogen around the half way mark. Tho when flowering plant din'tbuse much nitrogen they still need it