I started this, I'll take the heat for it, lol
When you said you have a ton of budding sites, you're absolutely right! They're everywhere too; up high close to the light, in the middle, buried deep inside the big bush, and down underneath where they're not getting enough light...and that is the problem; those sites are scattered in a pattern that makes a great deal more sense in an outdoor environment than the indoor one you've created for it.
Light from the sun is as strong at the tips as it is at the ground, so any light is good light outside. Your 600W bulb is a drastically different animal; what's almost unbearably strong at one foot is just right at two feet- and far too weak at three. At four feet it's better for illumination than cultivation. For a thouie, add one foot to the above numbers- but the basic premise remains; distance from the bulb is ALL IMPORTANT.
I could launch into a long winded treatise about the various ways a traditional overhead lighting system wastes light- that's for another post. Here, I want to make it clear that your current plateau does not have a thing to do with your cultivation skills; that plant is gorgeous! It isn't nutes, roots, shoots, temps... it wants for nothing and it shows. What's holding the plant back is its shape.
To be more precise, the most efficient shape is not just the one with the most bud sites; it's the one with the most budding sites in the goldilocks zone of ideal distance from the bulb. In the case of your 600, that would be between 18"-24" from a vertical bare bulb or add six inches for an adjust-a-wing.
In horizontal canopies with overhead lighting, you want to trim up the bottom a fair bit to rid the plant of excess larf. This reduces plant matter that's never going to see good light because of plant material above it- so get rid of it before it drains excess nutrients from buds that are in the light!
It's good that you spread the plant out early- as an adult plant, pull it even wider and thinner into a thick pancake shape. Your 600W bulb will penetrate a maximum of 6" into the canopy, so there's no point in letting the plant be thicker than that when measured along the axis that includes the bulb. Instead, make it BROAD- a plant that's four feet across can easily cover over 12ft² of surface area, and produce your pound.
To think about this vertically, just take the same flat trellis, turn it on its edge and roll it up around the bulb in the shape of a giant soda can. If the diameter of this cylinder were the goldilocks zone distance from the bulb in the middle, then the whole cylinder would be ideally placed, not just a few spots on a big flat trellis. For a 600W bulb, a cylinder four feet across would be an excellent choice. One bulb could adequately cover some 50ft² of trellis space, far more than it could over any flat plane.
I'm not suggesting your gardening skills are in any way inadequate; this thread is about you getting bigger yields for your girls, so I'm merely suggesting an option to achieve that.
The veg girls could be trained to climb a vertical trellis, much like pole beans, and spread out once they get taller. I'm training mine into a tall flat shape to better adapt to the trellis they'll be blooming in. A plant the size of yours could not only fill a trellis four feet tall by six feet across, but such a trellis would ensure that every bud it makes is basking in the very best light your bulb has to offer; not too close, not too far, never shaded, getting a breeze...
And that is how to make weight.
