Roots look like they are infected which is extremely common for rdwc grow with no water chiller and/or sterilizer/tea, bennies to combat the onset of pythium. I read through, but didn't see that you were using a sterilizer or a beneficial. If I missed that, please let me know. You are guaranteed to get root problems with water temps above 70 and no preventative solution. I see your ph is at 5.6 and you are using drops to measure. You should only use those as a backup and at least get a cheap ebay pen for $8 and some calibration solution, they do work. With the onset of pythium, it will naturally make the water more acidic lowering the ph and locking out key nutrients. Also, with ppm that low out of the tap, it's a good idea to add some calmag. Ph should be closer to 5.8 (slightly orange/golden color for drops).
You also need to fix your root problem asap. There are several opinions on this btw. My method which has worked several times is to inoculate the system with h202 at 3ml/gal. Then 24 hrs later, do it again. After the 2nd dose of h2o2, give it 24 hrs then change out your water and add hyrdoguard or brew a tea. I've heard of both working in higher water temp dwc applications, but to be honest, it's a lot easier if you have a water chiller to keep the water temps in the mid 60's. Teas and hydroguard are 'good' bacteria that basically consume the food source for the pythium, which prevents it from growing. If it's already prevalent in your system I would advise you to wipe it out first before adding beneficial bacteria's. From my experience it works and makes their job easier. H2o2 is nice because it evaporates rather quickly unlike bleach or other sterilizers. Those are the second option, to run a sterilizer like bleach, h2o2, zone, sm90. Each require regular applications. The bottom line is you must either sterilize or use beneficials to keep root rot under control. They are drooping because they cannot uptake water/nutes due to the roots drowning in rot or the ph of the water being low.
