Hi, I'm really sorry you lost two birds so close together! I wanted to share the story of our first flock because it's really easy to make this mistake. We had 3 girls, two lovely Buff Orps, and one EE. We delighted in giving them kitchen scraps and the kids would sneak them scratch. At 7 months one BO dropped dead at night. We thought it might be heat stress, so we buried her and were very sad. 3 weeks later our other BO died, this time it presented as a sudden lethargy, sleeping, not eating, drinking a little but sleeping on my lap, wrapped in a towel as I frantically tried to find a vet. She died as I ran her into the waiting room of the vet's office. We had a necropsy. She had died of fatty liver syndrome, her liver had ruptured and she bled out internally. She wasn't incredibly overweight, but it was enough. Neither bird made it to 8 months. It was really hard for us to process that we had in a way killed our chickens. We have gone forward now and feed 95% crumble or more per day. Our vet recommended keeping anything but food to less than 10% of their daily intake so that is what we do. We feed crumble, the girls don't like pellets much, and they can have whatever they free range for the few hours they are out, but crumble or not much else is the rule here now. In the summer they get a slice of watermelon or a corn on the cob to split between 6 chickens and only because it's so hot here that they stay in the run with the swamp cooler so it breaks up the boredom of sitting in the only cool spot.
I ended up doing a lot of reading on food. Most of the commercial ones are pretty similar. Somewhere on the forums there is a really good feed breakdown written by one of the very experienced people on here. We have a couple small flocks and making food would be complicated for us. I did some label reading and found one that I felt was decent and I supplement them with vitamins once per week, twice in the summer and then that's about it. They are happier and healthier now.