shouldn’t a few rounds from a mig 29 be able to bring a balloon down without any effort?

Nope. The U.S. did this to the U.S.S.R. constantly during the Cold War and the Soviets had less than a 5% success rate shooting down the balloons.

The US struggled with the one from China three years back. In the end it took an F-22 and a very expensive missile to get the job done.