That 7 gal or bigger pot rule (it's actually 15gal or bigger) is more intended for "no till" living soil, and for photo period plants or longer growing crops. In the photo period world 7 gal is about the minimum size you want to flower in. Autos don't live long enough to outgrow medium sized pots. I have yet to see an auto in a 5 gal pot live long enough for the roots to wrap around themselves several times, all the way up the pot. You should have no worries of running out of root space/food.
In a one gallon pot for example, at harvest, you can pull the dried soil out and see the roots wrapping around themselves. That's a sign that the roots are out of space and that the food in the soil is close to or already at depletion. In the photo period world, this is when we transplant into a bigger pot with with fresh soil/food. At some point in a one gallon pot, you will have to start feeding npk in some form or another, auto or not. Water only in small pots is rare. But you should not have that issue in home depot buckets.
The purpose of water only living soil is to feed the soil, not the plant, so that the soil can be reused over and over. So I wouldn't recommend adding any kind of bottled nutrient. I would use a compost tea to feed/increases the microbe life that break down the organic matter, which in in turn releases NPK for the plant to use as needed. At this point, one, maybe two applications of a compost tea or microbe inoculate like recharge, should be enough. You technically only have about a month or so to do, so not much else will be needed.
Congrats on getting this far and being disciplined enough to not just add stuff.
don't just start adding things because you read it somewhere,
