As far as I can tell, nobody's suggested taking a properly scientific approach here.

Each time you want to measure, pick a small set of different tests to do and record the results. You should get enough data to do some decent stats quite quickly. You can discard any that correlate well with shorter tests.

You should include in the tests subjective self-assessment (maybe you're better at guessing than you think) and real-world task performance (even if it's just "tried to do something and gave up", "succeeded in complex task" etc)

(PS, I'm not a psychologist! No doubt there's all kinds of subtle tricks I'm missing here, but I think the principle is sound)