Bro its miracle grow its not sand
Miracle grow generally is a "nutriented" soil, meaning it doesn't need any extra nutrients from you to grow. That being said, if this is one of your first times, adding nutrients to the plant WILL NOT increase its growth if its already getting fed from your soil because you'll likely end up causing nutrient lock out which is BAD. What I'm trying to say is, don't make the same mistake I made like, 3 times and try to add nutrients to miracle grow soil until its NEEDED.
That said, the plant is looking great and as with everyone else I will confirm, your highest yield will come from outside; there's no light source like the sun! If you want to increase the yield I would look into low stress training the plant and try to increase its overall "Surface area" by making it spread more across the horizontal plane instead of the vertical plane which plants like to grow on. (Because that's towards the light)
Last tip! Watering! Water when your soil is BONE DRY ABOUT 1 INCH DOWN from the base of the plant. Overwatering can cause a miriad of issues including overfertilization especially in soils like miracle grow because they have time release nutrients in them. Over watering stress looks like happy leaves except they look HEAVY for the plant to hold up and seem to lay down and not stand up straight like your pictures. Your plants look great and I'm just trying to help ya not make the same mistakes I made. Thanks for reading my wall of text buddy, happy grow! Ill be stoppin in to check up on ya!