How to fix small wobble in rear wheel?
Because you said you have disk brakes, it is unlikely that the unevenness is in the rim/tire. It must be something at the hub. Because you said it is visible at the rim, it is not a bent brake disk (unless you have both rim and disk bent). It is also unlikely to be loose bearing cups, though I would have checked them as well. When the wheel is not rotating, try to wobble it sideways with your hand. If it does wobble, tune up the hub.

The last possibility is that the wheel's axle does not sit properly in the dropouts, i.e. it is not installed correctly as the whole unit. Loosen the quick release skewer or nuts that hold the wheel and make sure that you've pressed it all the way in the frame's dropouts before closing skewer or tightening the nuts again.