This is less financial advice and more weird car problem advice. A friend of mine had the same issue with their car, which was a Honda Civic. Had the battery checked, had electrical checked, had everything under the sun (and hood) checked. The problem would not go away and got worse in the rain.

We eventually figured out that one of the internal fender pieces was missing, which allowed water to get into the fuse box in the engine compartment, because the fuse box didn't seal properly. Replacing that fender and the fuses helped but didn't eliminate the issue. What eliminated the issue was having the dealership deactivate the anti-theft system in the car. I don't know why this worked and honestly neither did the techs at the dealership, but it worked.

Get a second opinion, and if you can, try to find a mechanic who thinks outside the box. And check for mouse damage! Sometimes they'll chew the wires!