Just read back through your whole grow. I agree no N for the yellowing, up your calmag instead. By now you should be watering to run off 10-15%, that's part of why your medium pH was up.
You've done really well not pHing your water going in, but you should do that Everytime with pro mix. So since you're off the $35 dollar kick, spend another 50 to get a decent digital pH liquid probe.
If you are going to buy 12 lights, don't buy Chinese. You will either be sending them back for warranty repair or you're going to be replacing Parts yourself. And since you don't want to be building your own lights you don't want to be repairing Chinese lights.
With a 12 light purchase, go straight to a proven commercial agricultural light manufacturer, you'll get discounts.
I would advise you to take a hard look at the lumigrows pro 325, 5 year real warranty and you can link them all together and control the spectrum from one controller. They already have some research on cannabis specific adjustments. They are a 300 watt from the wall light that will get solid nugs 3 feet down, do I like to train my plants into a three foot ball. If possible.
Just grew two IPCBDs over a pound, 25 zips on the first I'll get a final on the other after curing.