Get a calcium-magnesium supplement and add it to your watering, red lines creeping up the stem and eventually into the veins of the leaves are often signs of a cal-mag deficiency.
Miracle grow soil is fine, as good as any other off the shelf brand and no worse. Specialty soil blends, especially those made specifically for cannabis are the only soils that might be significantly better.
I’m not sure of your watering technique. Often over watering or keeping the soil too moist too long or too often is the main cause of a cal-mag deficiency.
Most people flood and drain their soil. Slowly water from the top, evenly, until the soil is entirely saturated, get a feel for how heavy the container feels when it’s saturated with water. Then do not water at all until the container actually feels significantly lighter, actually as if all the water was gone. You should notice this change in weight well before a change in the plant shows it needs water. However letting it dry out so much that the leaves start to show the slightest sign of wilt in its leaves is another way people learn what the plant wants and when it needs it. If you do not over do it, this will not significantly harm or stress the plant and before you know it you will be able to read when your plant needs water just by glancing at its leaves, well before the average Joe could notice any sign of leaf wilt.