Newbie advise -caliber, lead free, altitude

I am starting to get into the hobby, and I've got a few questions I'm hoping y'all can help me out with. I'm looking for a decent springer gun in the $300 range. I'd like good power, a multi shot magazine, and decent quality. After a lot of YouTube and reading here and other forums, I'm almost settled on a Gamo Swarm Magnum in .22.

My goals here are:

1. Have fun -hence the desire for a multi round magazine

2. Hunt grouse and maybe gophers

3. Replicate as closely as possible a fire arm experience

Now for the constraints that make this more interesting, I'm planning on shooting lead free pellets at 7500ft (backyard)-9000ft (grouse) elevation. I know lead is much better in many ways, but between the chickens, kids, dog, and gardens, I'm committed to trying my best with lead free.

If I'm hoping to stay in the ~900ft/s sweet spot, should I move up to .25 cal to get heavier pellets, or will the power loss from elevation cancel out the higher velocity, lower mass of lead free? Or do I have everything wrong here?