Don't trust manufacturers and sellers of coops. They're trying to make money, not meet animal needs. There's no regulation when it comes to chicken coop manufacturing and selling. So humane animal husbandry is on you, not on them. The recommendation is 4 square feet of floor space per bird in the coop, and at least 10 in the run. You can cram them tighter than that, of course, but that can lead to problems.
This.
Granted that the plural of anecdote is not data, but over the years of reading this site even when I didn't have chickens and wasn't actively participating, it seems that the vast majority of health and behavior problems have insufficient space and insufficient ventilation as major contributing factors.

Another option would be a dog kennel for a run, they're usually pretty solid since they're intended to hold dogs.
Agreed. Chainlink dog kennel panels need to be wrapped in hardware cloth to deny access to weasels, rats, and grabby racoon paws, but they're made to keep strong animals from breaking through.