Alas, my goal remains the same - to produce curries that TRULY have the depth of flavour, savouriness and smell, typical of those produced by decent BIRs, rather than simply nice, tasty, curries.
I will continue to pursue the combination of ingredients (mainly in the base) and techniques (what are these?) required to achieve this accordingly. So far, none do.
My focus will remain on the base (where I still believe "the secret" lies) and any other additional ingredients (e.g. spicy oil, stock) that may be required to achieve my goal.
Based on Haldi's recent comments, I will be trying to replicate the spicy oil, which is undoubtedly produced naturally in BIRs in the course of their daily activities, perhaps to also use in preparing the base. It seems to me that, although I've found that onion paste produces no discernible difference in my curries, it may indicate a means (i.e. fried onions, garlic, spices, etc) by which to produce spicy oil for cooking with.