But wait, so you say its ph right. Pfffffff. Ok.
Just because I was able to identify the problem and be able to correct it immediately does not mean I did not think about other factors. The factors you mentioned were not worth mentioning because they were not and are not part of the problem. If you knew what you were talking about you would realize the problem is associated with the pH and the plant being unable to uptake the nutrients the plant needs/desires.
It is the pH.
Solution of 5.2 - 5.5 and then settling at 6.0.. you will see those red stems in many strains. 5.2 is 80-times more acidic than 6.0. It's not a little bit of a difference, its a lot. I would slowly work UP to 6.2-6.5 and see if the solution stays there (ppm's coming down with less pH drift) or if it drifts back to 6.0. The red stem issue should go away (on new leaves) and the buds will fill in more because the plant will be jamming.
The problem with nft is changes to the system affect everything in the system. Sometimes having separate systems is helpful to learn and experiment without affecting the entire crop. All of your eggs are in one basket so go slowly with pH adjustments. Nutrients of 750ppm are also a little on the low side but I would adjust pH first, and then increase the ppms. Get the pH resolved and then you can feed them more.