I know next to nothing about this topic, but wouldn't "ladder stocking" of the grow-out BG perhaps work better?
As you move some 1.5-2" BG over to the main pond, then the population pressure on the food and space resources in the grow-out pond should be somewhat reduced. Would that help the remaining BG population more rapidly move to the 3-3.5" range?
You could even do several partial transfers.
That way you get lots of BG over to the main pond, and also get a significant number of larger BG over to the main pond.
(Just thinking out loud and reading what our experts say.)
