Question how much it actually matters. A collapsed middle might just make the cake more dense and tasty. Oven thermometers are cheap and can check your temp, but sinking happens for lots of reasons. Overmixing the batter or pulling it out too soon can cause that. Did you try a piece yet to see if it still tastes good? Why get worked up over looks if it's still edible?
Seriously? Looks totally matter though. If I'm bringing a cake to a party and it looks like a crater, people judge it before they even taste it. Presentation is part of the experience, and a sunken middle screams "kitchen fail" even if it's moist.