I don't think you're missing anything obvious - this is actually a tricky problem. : D
The issue, of course, is that product pricing is enacted at the line item unit price level where all we have is a reference to the particular SKU the line item represents, not the node ID that may group related SKUs together. I'm honestly not sure this is even worth pursuing with the default Rules conditions / actions, because the logic is just complicated enough that it would take longer to do that than to code a custom condition.
If I were developing a site with such a requirement (and I've come close! I actually have this coming up in a project slated to start in July), I'd create a custom Rules condition that would examine all product line items on a cart to find the total quantity of all products added to the cart through the same Add to Cart form. In pseudo code, it would look something like:
1. Look in the line item's data array to find the reference to the entity containing the grouping product reference field. This is stored in a context array that's intended to be used to rebuild the form if necessary.
2. Extract the related product IDs from the referencing entity.
3. Loop over all the line items on the current line item's order looking for product line items that reference any of those product IDs.
4. Sum the quantities of all matching line items.
5. Compare the final sum against a parameter from the condition using a configurable operator.
Not sure how much that helps, but suffice it to say I can't imagine trying to make that all work through the Rules UI.
