Eggs haven't been studied too much in poultry science probably because it is counter-productive to egg farms to feed eggs back to the birds, but my instincts tell me eggs are the "perfect" food for chicks. I was advised to top my poult food with boiled eggs, though I wasn't ever told how much. My birds LOVE scrambled eggs ... they get some most days, and I always mix scrambled eggs into the baby food. But I don't worry about how much because I don't have any info on quantities.
Lots of people recommend liver. Raw, fresh, frozen, dried ... as long as the bites are small enough ... I've read recommendations for all of those. But again, no advice about portions. Liver has tons if nutrition.
There are also chick vitamins & probiotics to put in their water. The probiotics and some of the vitamins would be redundant if you're getting FF into the chicks.
Some people recommend cat kibble (high meat kinds, and some people recommend against the kinds with poultry, so that leaves fish-based cat food as the "safest" choice) ... you can soak that in milk or water to soften it and/or stir it into the FF. I do this occasionally for all my birds ... stir in a tiny amount if cat kibble the night before the FF gets served (just long enough to let it absorb some moisture to soften, but not so long before that it might get stinky ... just my preference). It keeps the birds super interested in the FF. My feed mill recommends this very highly.
If you can get your hands on fish meal that's a great source of nutrition for poultry. You have to watch how much goes into layers or older meat birds so the meat & eggs don't pick up a fishy flavor, but there is less concern about that with chicks.
I've been curious about powdered dairy proteins for poultry -- dairy used to be very popular for poultry. I'd think that would be a great protein source, and wouldn't flavor meat or eggs, but it's expensive and current research is sketchy.