You were lucky enough or smart enough to pick a bike with a decent tire to start off with. Most tires that come stock with most bikes in that price range are pretty juncy but that GT is well speced and those Schwalbes are an excellent choice.

The cracking sidewalls are usually more a symptom of lots of UV exposure coupled with running the tires under inflated so buying a more expensive tire isn`t going to change that. On the other hand - if you buy a cheaper tire than what you already have you`ll probably wear the tread out in a couple months based on the milage you posted.