With UA being a non-starter for you, AA is probably the best option. However:

--Flying to ATL is a connection in ORD or DFW
--Flying to NYC is a connection in ORD or DFW
--Flying to LAX may well become a connection in DFW, because DEN-LAX is flown by AA Eagle, and the big planned cuts in bankruptcy court will likely put non-hub flying like DEN-LAX in danger

At about 50k per year you'll just make mid-tier status. If you occasionally fly somebody else to get a nonstop...perhaps to have more time with the family, or to make your business plans "work out", or simply because of the wear and tear on you...you'll fall short of the 50k threshold and be in the bottom FF tier. From what I've heard, the big airlines will only comp lowest-status FF status to Frontier Summit, so you might never hit mid-tier.

The downside of the big international airlines is that it's very tough to be "anybody" with them unless you rack up the big international miles. Lower-tier status is much less beneficial, especially if you don't quite hit the threshold to achieve mid-tier.

On Frontier, Summit is it. There are benefits that a big airline can give elites (even low-tier elites) that Frontier cannot, such the possibility of upgrading to a true first-class seat. However when you need a rule bent, or you need to get through to a res agent, or you need some extra TLC in rebooking when your flight is canceled, you're a big fish in a small pond with Frontier. When you need a big airline to come through for you, you're a little fish in a small pond.