Welcome to FT! ^

Given your travel patterns and being based out of IAD, I'm afraid United will still be the best choice even with all its shortcomings. Any other airline/alliance will require lots of extra connections and hassle.

Some airlines will match your status. No airline will match your miles.

Most other carriers have much better lounges. I'm pretty sure you can join them directly, or you can get Priority Pass which gives you access to over 500 lounges. The problem is, if you continue flying United, most of these lounges will be very inconvenient (located in different terminals, etc).

Do you fly mostly Y (economy) or C (business)? If the latter, you should avoid United like the plague on international itineraries and fly *A partners instead. You should also credit your miles to some other *A program. But if you fly mostly Y, Mileage Plus is probably your best bet.