He made major contributions to statistics. The Wald test in statistics is named after him (https://en.wikipedia.org/wiki/Wald_test). I think he was also a forerunner of decision theory and invented an early form of the minimax principle:

Wald, A. (1939). Contributions to the theory of statistical estimation and testing hypotheses. The Annals of Mathematics, 10(4), 299-326.

Few weeks of intense reading to understand a whole semester's worth of mathematics... I'd also like to have that skill.

Perhaps being homeschooled helped acquire such skill.

The article covers so much -- and it doesn't even mention the "Wald estimator," which is part of the foundation of the modern (causal-inference-oriented) view of the method of instrumental variables.

...died in 1950 aged 48 in a plane crash on an Indian lecture tour.

Add to other mathematicians who would have been even more prominent if they hadn't died before their time: Galois, Ramanujan, Turing...

(when following Wald's work on rearranging series)

This is a nice riddle:

Is summing the series:

1, -1, 1, 1,....

0 = (1-1)+(1-1)+(1-1)....=0+0+0...

or

1 = 1+(-1+1)+(-1+1)+(-1+1)....=1+0+0+0...