Is that the SFxY? Or the Chipper?
When you ask "will this ever fill in?" I don't see what you are asking about. I don't know the plants you are using, but everything looks fine to me. Looks really good actually (although the pic quality may be making them look super frosty)
See how most of your fans in that pic are splayed out, strong, facing the light (If the light is at a different angle, the leaves will try to be perpendicular to it) That is a plant happy in it's current environment.
Nutrient wise, I can't tell if you are flushing and that is yellowing the leaves, or if it's just the pics. But temp and humidity....from the pics, they look happy. I don't know what week you are at, but it looks like you may want to drop the rh soon. Unfortunately, at the end of flower, it's to our advantage if we make the plants a tiny bit unhappy by drying out the atmosphere.
The more I grow, the more I see that, environment is everything in this game. You need the light, the genetics, the right nutes etc...you have to have all that down too; (<proper use of the semicolon) but I feel like that part is easy/cheap enough to buy/learn (improper use of the /). But to run the perfect environment the entire run, man, that's a lot of time, energy, planning, and $. I did well this run...but I had at least 20 days during flower that were hotter than I wanted.