Not sure about the advice to feed carbohydrates over protein, particularly with history of laminitis!
The thing that will increase milk yield is forage - plenty grass, quality hay, or as suggested, alfalfa, ready grass type feeds. Carbohydrates can increase fat percentage in milk, but with risk of grain overload and laminitis.
I think it sounds like you're doing right by keeping a close eye and topping up with a bottle.
I also think that you are better to have slightly underfed than overfed before kidding as much less risk to mum and babies, you can always supplement feed for mum and babies afterwards, but laminitis, ketosis, trouble birthing large kids can all be fatal!