So sorry for your losses. It is never easy.
We got three orders from Meyer last year, and lost 3 chicks of the same breed. They arrived the morning after they were shipped. The first died just 8 hours after arrival. The second the next day, and the final one on day 6. Meyer shipped replacements for the first two, but not the final one, but I did call them about the final death because it seemed like a problem with one breed batch. All other chicks in that order were healthy, and we have only lost one to egg peritonitis at age 6 1/2 months.
Of the second order, 3 weeks later, one of seven had pasty butt when it arrived (day after shipping again). We spent hours over 10 days on that chick, he turned out to be our roo for that order. He is a healthy rooster today. Without the diligence on our part he would have been a goner. All of that order are healthy today.
The final order was and still is healthy.
I believe that when a young chick dies of unknown causes it is often something related to the health of the parents, the incubation, and/or internal health, that on the surface is not evident.
Your care sound right on target. Good luck with the rest of your flock.