Help! Recurring respiratory infection

Hi there. I have a 6 year old male corn snake who has been experiencing a lot of respiratory problems within the past year. About a year ago, he began making puffing sounds and wheezing, so I took him to the vet and he had many tests and injection treatments to cure his respiratory infection. They even did a nasal flush. He also had pinworms so he got medication for that. The whole process to get him better took a couple months and it put him under a lot of stress. I have a day lamp that keeps his tank at about 80 degrees, and a night lamp that stays about 75 degrees. I live in Arizona and the climate is dry, so I try to spray his tank with water to keep the humidity up as much as I can, usually at about 50%. He eats well and sheds regularly. His bedding is aspen shavings, which I know is not a negative factor because his first respiratory infection began while I was using coconut fiber substrate.
With all this being said, within the past couple days he has started puffing and sneezing again, and today started wheezing a lot. He is much more active than normal, just as he was last time this happened. I am going to make an appointment with the vet this week, but I am desperate for anyone else's opinion about why he is getting these recurring respiratory infections. Any advice on ideal temperature and humidity, or anything else you may think is contributing? I've been reading a lot of people say an undertank heat pad is the way to go, but is my lack of one honestly causing these issues? Thank you so much in advance.

He should have an UTH and it should be about 85 on his hot end, he is kept too cool and that will lower his immune system which is most likely hwy you are having these issues. He can't properly thermoregulate. I can't guarantee it is but it'd be my suspicion...

I would also put in a moist hide as well if you have a dry climate, misting only helps so much and can also create mold etc with the aspen which can also lead to URI if you do it daily...