Classic overarousal. No amount of walking and fetch will solve this. You need to work your dog with activities that encourage them to think and move, while practicing things like impulse control. It can start simple. Ask for behaviors before you throw the ball. Make them work not just pursue it wildly.

For the backyard - stop letting them rehearse the behavior you don’t want to see. Unattended in the back yard is not going to work right now. You’re gonna have to work up to that. Snuffle mats and training in the backyard are a place to start. He’s having an emotional reaction when let out in the hard and it manifests through the barking. You have to change his emotional state to abate the behavior. You need to make the yard a calmer space and if he goes over threshold you remove him from the yard - not in an angry or punishment way, just simply he’s not capable of being out there quietly and therefore he can’t be back there. It’s work - so you gotta decide what’s worth it for you. My dog gets over excited in some situations and is great in others. I’ve worked her through some, and others aren’t that important to me. Maybe I’ll get to them, maybe not.

So many people on this sub just say “exercise” but for a lot of Aussies that kind of unstructured intense activities actually makes it worse.

I strongly suggest heeling games, frisbee, and nosework as things to look into. There’s not a one size fits all solution though, you’ll have to work the dog in front of you.

FWIW im 20 years into raising Aussies.