> well-resourced students are more likely to get accepted into, and graduate from, elite schools. Happy to back that claim up with some research if you like.

We agree on this, but it's also a major confounder for observational word gap studies. And though we try, it's very difficult to fully control for confounders.

The holy grail would be interventions that can improve adult intelligence but unfortunately a lot of interventions that look promising in short term studies see their benefits disappear by the time the children reach adulthood. There don't appear to be any long term studies on this yet (I know this is a big ask).