.

It isn't that the MG soil is shitty, the time release nutrients are just unpredictable. From your description, it sounds like you have nute burn, both too much nitrogen (crispy leaf tips) and too much phosphorus (purpling of leaves). To be safe, flush the plants with 3 times the volume of water as your pot holds (example: for 1 gal pot, flush with 3 gal water) and let the soil dry out for a few days. This will not only help flush the built up fertilizer salts, it will help leach out the time release fertilizer pellets. Once they are dry, add nutes at 1/4 strength. Your Ph doesn't sound too bad, just try to keep it stable.