Never ever cut a double layered coat of a dog, unless absolutely necessary (e.g for surgery).
Instead make sure you get all the old hair out. A groomer might be a good idea, being able to give you tips on this.
Many people don't understand how the two layers of coat on dogs work. They actually help keeping the dog's temperature down. Trimming the fur impacts this built-in temperature control and can even be harmful, e.g. by exposing the dog's skin to UV rays. (More elaborate details can be found on the internet, e.g. here)
Our Huskies have no problem with temperatures in summer etc. they just need way to get water or some shadow, if they want. I bet it's similar for your Corgi. In addition, we've often met self-proclaimed experts complaining about us owning our dogs in Europe, while having trimmed down dogs that absolutely fail to withstand the hot weather.