Where are you??? 15% protein, if not supplimented heavily with AAs as they do in the EU, is low protein for a US feed, which is further reduced by the scratch, likely only 8-9% protein.
Recommend giving up the scratch entire, just throw more feed at them. Most birds will scratch for feed like they will scratch for scratch, and at least you know its a complete diet.
You can give small amounts of kitchen scraps, too. Most veggie peels, egg shells, small fruits or veggies gone soft, etc. Try to keep the amount of stale bread, uneaten pasta, etc down.
As "treats" go, you have a very small flock. Bird seed contains lots of, well, seeds - which are generally ok to decent in protein but typically quite high in fat. While more protein is good, more fat is bad - and generally, the more fat is much more bad than the more protein is good. Same with BSFL, Mealworms, etc. Look on the package for the nutrition label. Some of the dried varieties can be in the 25-40% protein range, but also up to 35-45% fat. SO giving just 10% of the daily diet as those concentrated nutrition sources can essentially double their daily fat intake!
Of the two, I'd choose the bird seed over the worms/BSFL, not only because its cheaper, but because its less nutritionally concentrated, so there is less risk of dietary imbalance long term, and its easier to portion control Of course, i choose neither - and simply throw feed to my birds as scraps, which remains the best option (in my view).
