m0useanswered grow question 3 years ago I have been using a cheep PH pen for 2 years now and as long as I calibrate it every so often and keep it in the storage solution its worked well for me.
I do dream of a blue-lab one day though.
Leaves look a bit nutrient burned but also spotted like they want calcium to me.
not familiar with your nutrients. double check their primary and secondary macro ratios of n-p-k and ca-mg-s to make sure its a full feed.
I don't think your plants should be fading yet its got lots of flower time left.
Good Luck!