Your air supply is too small for anything that would be a quality impact. Cordless is the way to go. I actually have a couple of evolutions of cordless tools. Beaters being a 18V Craftsman (Dewalt built) set, and that impact wrench has been hard to beat! Almost to the point I cannot recall the last time I had to pull out the big IR air gun I have! That said, I also have Makita XLR tools and recently was in a pinch and needed a stubby impact gun so I bought their compact 1/2". The thing IS A BEAST! I'm quite certain they're conservative of their ratings because I broke lignuts off a tractor when the torque spec is 340 ft-lbs! Nice, compact form factor and it pairs well with their cordless ratchet I also have.
There are some great reviews on YouTube, including some by RVers that are also mechanics doing comparisons you could search out. I think any mainstream cordless tool maker has you covered at the point.