hey there BEZ, well actually it's a bunch of things some simple and some complicated. First thing is the pH which started the mess, glad you're controlling that now, just stay on top of it : it's the #1 hardest thing. Second there was some over watering, in such a big pot you should be watering twice a week maximum more or less... there's a fair amount of water logging happening, which is when water gets retained by the medium causing problems. so it's caused two problems, first problem is root problems : you can tell from the leaves beging to start cupping. this indicates root problems. Second problem is the water breaks down the nutes in the soil and makes them available, so suddenly the seedling ggets a bunch of nutes which leads to lock out. There's a few ways to fix this. First off you can add maybe some enzymes to try to break down the problem roots. Second and most important way is to water less, and stick to a strick wet / dry cycle using the lifting method to see when you need to water. you can probably cut out all the nutes, especially the first three weeks, no reason a seedling needs an injection of liquid fertiliser in soil. So basically a very common a fixable problem. Hope this helps ! 🚀