I'm so very sorry for your loss and so glad your birds are safe. You were very lucky. High winds could have spread the flames to other structures.
Hindsight is always wonderful but the truth of the matter is that there for the grace of God go all of us. Our coop is a converted old shed. The key word is old but it is insured as an outbuilding. Something to consider with your next coop especially if you go with a shed or permanent building.
Also, I had rambunctious bantam roosters break out a regular light in our coop. Why they decided to fly as high as the ceiling, I'll never know but it taught me a lesson. Wire cages around all light bulbs.
Good luck with your future build. Lousy time of the year to be having to build, but you have some good solid ideas to build on plus you now know how to fireproof your new coop.