Promix is not the problem. The dry back pattern shows severe underwatering is one of them. You need to water All of your medium and then allow it to dry back and repeat.
I use promix
If watered properly, MG works fixing great. You did and do not water properly ever based on the evidence you have supplied. Water the whole damn medium instead of waterboarding the central roots constantly.
I swear you're trolling us. Can't see how you could hate your plants so much to...