Yorkshire pudding - how do the proportions of the ingredient affect shape, texture and taste?
I take it you didn't read Kenji's Serious Eats column The Science of the Best Yorkshire Puddings which is linked from the recipe column.

It discusses his experiments with various factors affecting Yorkshire puddings addressing some of your points including liquid (eggs + milk + water) to flour ratios and amounts of fats (yolks + milk) as well as technique.  The focus is on balancing expansion, crispness, and the hollow with some mention of richness (fats).

[EDIT]

Major points:


Let rest overnight ("Resting your batter is the single most important step you can take to improving Yorkshire pudding and popovers.");
Starting with colder batter gives denser, more cup shaped; warmer batter gives taller, crispier with a hollow core;
Water added to milk gives a crisper pudding (not needed for skim/1% milk);
Hydration at least 200% and no more than 300% (266% in his recipe) - higher hydration gives "a pudding shaped cracker", lower gives deeper cupping;
Pre-heating muffin/popover tins not required, but essential for cast iron pans;
You can open the oven without ruining it;
Beef drippings are best.