a machete works well with snakes
A PCP is a big buy-in for your first air rifle. I think an HW50 is still perfect for what you’ll be doing. At 7-12 ft distance I think a .177 HW30 is plenty unless rattle snakes are tougher than I thought.
Snakes aren’t that hard to kill, shot ‘em dead with pistols doing 3-4 fpme. I think a light springer rifle will be just fine.
Even a lightly charged pumper...