I have been using the Go Box for years.
It's intended to be used with soil and that has no nutes, but I've always used potting soil that has nutes.
If there are nutes, you hardly use the two big bottles at all.
There's a number of good brands, and prob some local mixes that would also be good. Most will be better than MG.
What does your local grow store have? Pro-Mix has a good rep, haven't tried it. I am using Roots Original now, and it's good, but not ideal.
I've heard that their other potting soils are better. I've used and liked Dr Earth Home Grown (only the Home Grown, which is the top of their line)
but it's a little expensive, and I'd put it in a middle tier with Roots Original, better than MG, perhaps not as good as the others.
I add perlite to any mix I use (although some already have excellent drainage).
And there are a ton of recipes, both here and across the net, for making your own potting soil. If you are using a Go Box, you aren't growing a lot of plants, so maybe a recipe for 100 pounds of potting soil is overkill at this stage.
Start nutes at 1/4 dose. The exceptions to that are the seaweed, humic and cal/mag, where you want to start at 1/2 dose.
Some plants like to nibble gently at nutes, others are like a starving man at a buffet.
You will have to watch the plant and let it tell you what it wants.
Learn about burn (and other problems, esp. overwatering) and watch for them. A little burn is not bad, but shocking the plant is not at all what you want. I try to avoid burn completely. Think like a gardener, everything they need, but nothing more.