As you pointed out, IQ is not usable for research across time or across countries or cultures. You can only compare individuals in the same time and same population.
Honestly, iq tests are pointless unless you're selecting for people that are willing to invest time into worthless knowledge in order to win in a contest.
Which is a valid criteria, considering that these are the most motivated people... Its just not "intelligence", though that itself is just as pointless to measure.
I get the sense from the comment about BMI and "axe grinding" that you're really trying to make a dig at what conservative leaning people might call "SJWs". If that's the case, you're doing yourself a disservice by letting ideology obscure the very real scientific problems with IQ testing.
As I said before, the iq test mainly measures how much the participant is motivated... Or so many other reasons which are loosely correlated with professional success such as enjoying puzzles and figuring things out etc.
It just doesn't say anything about "intelligence", and measuring that is pointless, because it's not even possible to clearly define it... like so many terms, there are as many definitions for it as there are people in the room.
But even if you use the official definition of it being the ability to apply knowledge I'd still disagree with the usual iq tests measuring that. They're puzzles at best and measuring how someone can apply knowledge is not that easy to standardize.
For a population studies this does not matter at all.
For individuals, practicing for IQ test might help a little but there seems to be a clear limit IQ limit that individuals can't overcome with practice.
