I keep loosing leaves to some sort of nasty necrosis. It affects leaves at all levels, small and big randomly. I had to remove a lot of dying foliage

Just water until the pH of the run off sorts out.. simples. As its showing that the feed you are putting in is coming back out because too much feed so reduce your feed after a few days only water.. you need to connect with the plant she will tell you if you change 1 thing at a time you will know what's he problem.. always give low amounts of anything until you know she can handle it...

My opinion: "I have high runoff EC3.3" sounds like nutrient lockout brother. 3.3 is insane! and the plant probably can't uptake phosphor anymore and this is why you got this problem. I think there is only one option. Flushing with pH water until the run off is at least 2. maybe even lower. The plant definetly shows early signs of nutrient burn (leaf tips) and we can see there is necrosis so the plant can't uptake despite there is a lot. that's why i would have guesses nutrient lockout. good luck brother!

Iv sent you a DM mate on my thoughts but Asnoriu seems to think similar to me.

Lack of phosphorus is main reason for necrosis , that bottom leaf could rub other pot, but i would review feeding ...