Valley fever, allergies, infection, or foreign body

My 4 yr old lab has been coughing nonstop for about 2 months now. We have been traveling all over the west coast states including az Utah

On the last day of our trip he started coughing and dry hacking so i made an appt back with our primary in Utah.

Got on antibiotics and did a valley fever test ab a week after symtoms started. Everything came back negative and the abx didn't make a difference. We ended up going back in and doing one of those look up the nose with a light sedation (not with a camera). Found nothing.

We've been to two different vets and I'm losing my mind trying to figure out what I should do. I'm also running my wallet into the ground doing all of this.

Do we do

1. imaging (MRI OR ULTRASOUND) maybe even X ray
2. Get a camera procedure and look deeper into the nose
3. Retest for valley fever (since we tested after a week maybe false negative )

Besides the coughing (phlegm and congestion) and nose licking there's no other symptoms.
We're getting deep into the thousands of dollars and I can't afford to keep doing this and find nothing and get told I need another thing.

He has been on 3 rounds of antibiotics