I glanced through the thread but didn't see anywhere that said you raised the lights once the intensity got raised from 400w-750w? This is likely your issue, if the 750w light is in the same height as the 400w light then that is likely what your issue is. Interesting thing about light burn is that you'll notice symptoms of nutrient deficiency that is actually related to your light being too close. When light burn happens, most people think it's a nitrogen deficiency or even toxicity because you start getting "the claw" and leaves start to yellow on you. However, it is only a nitrogen issue if it starts from the bottom of the plant and works it's way upward.
From what I can see in all of the photos you've provided most of the symptoms appear on the upper half of the plant as opposed to the bottom. Deficiencies of macro nutrients tend to start from the bottom and work their way upward, however if you take a look at all of the pictures you posted you'll notice most of your symptoms are at the top of the plant which tells me the issue is your light being too close. The fact that this wasn't an issue before you upgraded your light confirms this for me.
I keep my 600w lights around 24 inches above the canopy, give or take. I found this one out the hard way, but you can have light burn even if your canopy temps are a perfect 75-77 degrees. While the actual heat coming from the light won't cause you issues if your temps are where they should be, it's still very much possible to give your plant too much light. I've seen a lot of new LED growers have this issue, they figure since the lights are so much cooler they can be placed closer to the plants as a result.. however they still end up getting light burned despite the temps being where they should be.
Fortunately you have a couple of options as to how to fix this. The best move would to just raise the lights so you can make full use of all 750 watts during flower.. however I notice you're in a tent and it may not be possible for you to raise the lights up any more. If you're unable to raise the lights then grab yourself a dimmable ballast and run it at 50-75%, whichever setting works best for you. After you've fixed the issue with the lights make yourself a foliar feed of silica and kelp meal as that will help with the stress.