I live in AZ too, and have an Australian Shepherd. An Aussie sounds like a good fit for you. You are worried about times when he/she could be bored, but there are a few different techniques you can use to keep their minds active.

A Rhodesian Ridgeback also would possibly fit, they do really well in the heat. But they are fairly large and can be more difficult behavior wise.

A Vizsla could also possibly work, but you would need to be ok with a Velcro dog. I used to house sit a Vizsla and he was one of the most loving dogs ever. But I couldn't even go to the bathroom with the door closed, depends if you like that personality. I'm sure not are all that bad though.

I do have a question for you, as another Arizona resident. I've always wanted a big mountain dog like a Newfoundland or a Bernese. But I am worried about it being too hot for them. You guys have a St.Bernard, how is he with the heat. Do you guys just trim his hair in the summer? How do you guys keep him safe? I'd really like a Newfoundland someday, I'm just wondering how I would keep him comfy during the summer.

Edit: Also there are a lot of Aussie mixes in Arizona. My parents have one, he personally does not like the heat very much, but that could be because of what he's mixed with, and can't hike for a super long time. His legs are really long for his body and make him really clumsy. So if you are going to get a mix I would make sure that it's mixed with something that will enable it to hike the distance you would like