I made a front air dam on my truck mostly because the factory chin spoiler had been destroyed by hitting snowbanks, and also because I really wanted to build an air dam. It did not affect the gas mileage.
I did none of the other things on your list because of cost (lowering kit), side effects (smaller rear view mirrors, lowering), or lack of effectiveness (everything on your list except possibly rear view mirrors).
Here is the list of what I tried, and the results:
https://ecomodder.com/forum/showthre...yon-17070.html. You have a smaller truck and live in a warmer climate, so should be able to get better results than I got with my truck.