Yeah, there is a whole lot going on.
I don’t understand exactly what you mean by that they don’t grow nice buds.
I also won’t try to figure out what all your different nutrient mixes mean. I can’t, I just can not try and learn every single nutrient mix and schedule out there, and besides I keep it real simple following basically the simple Lucas formula, with only slight tweaks during veg vs. bloom, regardless of what nutrient “brand” I am using. The EC/PPM of your nutrients might be very low, 280PPM is very low, except for clones and seedlings.
If your roots don’t get enough oxygen or enough nutrients, both of these things will reduce root mass. The mycorrhizae from the great white should help with both, the roots getting more oxygen and nutrients, but something else might also be going on.
Another thing is, I don’t really flower in soil, so I can’t really help you there. I flower in DWC, and in DWC, you almost never hear of a problem developing root mass.
I’ll tag @latewood to this post, he has a lot more experience with soil-less mixes like yours and might have some additional insight.
One other note as you are using a LED system. A 300 watt LED might not actually consume 300 watts of power, depending on how the manufacturer labels things. However Advanced usually does use pretty darn close to what they claim, and so if it says 300 watts, the power consumed will usually be very close as opposed to a cheaper LED that uses, say, 100 x 3 watt LEDs and is actually using only about 150 watts of real power, but calls itself a 300 watt because it uses 100 LED’s that have a max rating of 3 watts each even though they are never in reality run at their maximum wattage.
And so even if your light is really using 300 watts of actual power, even with a really good light like Advanced LEDs, you will not get the actual performance of a 600 watt HPS, sorry, it just doesn’t work like that in reality.
In reality, maybe you can get away with using 35 watts of LEDs for every 50 watts of HPS you are trying to match. As you can clearly see this is not a 50% ratio of 300 watts of LED equaling a 600 watt HPS. A 300 watt LED by Advanced will only maybe come close to equaling about 500 watts of HPS, if that, and even in Advanced’s tutorials they admit that it is about a minimum of 35 watts of LED per square foot of canopy, or 50 watts of HPS for every square foot of canopy, and so you can clearly see, it seems you might be severely under lighted. This alone will severely restrict your flower’s development.