Trying to figure out how to approach this to help you understand. One morning in Arkansas the temperature was -4 Fahrenheit. 4 degrees below zero. I opened the pop door and let the chickens decide what they wanted to do. They immediately went outside to enjoy the weather. -4 F is a lot colder than 30 F.
Cold weather provides two different risks for chickens. They wear a down coat that provides great insulation by trapping tiny pockets of air. If that trapped air is released, they lose that insulation and can feel cold. If a breeze ruffles their feathers enough those air pockets can be released and they can get cold. They need protection from breezes strong enough to ruffle their feathers. If a stiff wind had been blowing that morning those chickens would not have been out there but it was dead calm.
The other risk is frostbite. This is a bit more complicated. Any time the temperature is below freezing they can experience freezing. But a contributing factor is moisture in the air. If the air has high humidity you can get frostbite at a higher temperature than if the air is dry. That morning in Arkansas the air was really dry.
In a coop you can get moisture from their breath, their poop, and any open water sources that are not frozen. You get rid of that moisture with ventilation.
So your goal is to get ventilation without a breeze hitting them. You can accomplish that by have openings above their head when they are on the roost. That is another advantage of having a walk-in coop, more room to have high ventilation. If your coldest extreme temperature is 30 F you don't have a cold problem. I suspect though that your record low is a little lower. It is the extremes where you will get in trouble, not the averages.
The heat will be more of a problem for you. Chickens can normally handle temperatures into the mid-90's OK but shade can help. If it gets above 100 F they can be in trouble. Your temperatures will probably drop at night but you still need good ventilation in your coop to help it cool off. In addition to having lots of good ventilation up high, having a vent down low can help a lot. Cool air is heavier than warm air, that's why warm air rises. If you have a vent into your coop down low on the shady side where the air is cooler you can get some decent air movement. Cover your vents with hardware cloth to keep predators out.
People keep chickens on the Equator and way up north. With your temperature ranges you should not have nearly as much trouble as they do. Good luck!