Though I am a BIG proponent of using bennies is every situation, your inability to locate the needed products (Great White, Hydro Guard) and a few other things, there won't be enough time to try this or that and hope. That said, I'm with everyone else in that you have pythium, and because all your plants are in the same system, they all have it and can possibly die if the damage isn't stopped. That said, you should mentally prepare to lose this grow.
Your best bet now is H2O2 and discontinue the voodoo while using the H2O2 for the rest of the round. IDK if it would be wise to remove all of that dead root material, the stuff that easily breaks away before the H2O2 bath, so confirm that with someone else, but you gotta do what you can to clean the mess as best you can, then the bath, then mix the H2O2 in with your nutes for continued treatment. It might even be wise to prepare 3 day reservoirs so that dead matter that falls off can be flushed out so not to reinfect. FWIW, I did a little experiement like 5 years ago with a little DIY aeroponic system and was using H2O2 in the res and still got pythium, and all my clones in it died. That shit is BAD.
For future grows, I highly recommend expanding your knowledge of the wide variety of beneficial bacteria. The more the merrier. I regularly use Heisenberg Tea in my grow, twice weekly in fact as a preventative maintenance kind of thing. Because I use synthetic nutes in rockwool, my bennies themselves die and are washed out regularly, but their strength is that they eat pythium before they die and my roots are always in fantastic shape. You can also keep great roots, but you need to acquire some materials first.
Here's a FANTASTIC old thread all about this very topic and a wide variety of products you can accumulate and use. The author takes into account for people that don't live in the USA so you'll likely read a few products that you'll be able to pick up and use.
https://www.rollitup.org/t/dwc-root-slime-cure-aka-how-to-breed-beneficial-microbes.361430/
