It sounds pretty healthy to me. At 6 months they are either still growing or a bit lean due to being teenagers. Are you giving unlimited hay?
Getting a bunny used to a higher fat content is not the best idea for her long term health. If anything, she should bulk up on the hay so she can have regular elimination while getting the good nutrients more than the fats. More greens is fine, but don’t bother to add sugars or fats now. 6 lbs at 6 mos. is healthy, heavy for a smaller breed and growing normally for a larger one.

I once had a vet call my mini rex Spockie, who weighed in at 5.5 lbs. at about a year old, a “lean, mean bunny machine”. He was on the lower side of his weight category, but she said he was getting good exercise, eating and pooping well, so just younger. Sure enough, he got heavier after a couple of years. I would not worry.
Watch for eating habits and poos. If any of that changes or stops, have the vet check her. When was her last exam? If it’s been over a year, it may be time for a check up.