Oh banana cream pie, and lemon meringue were toughies. Its doubly hard because what I taste that makes these flavors what I want is not always what the other guy finds satisfying in his recipe since DIY chases the elusive. The BCP from the vendor might be spot on but if you want to make your own to save money getting yours to match can be a headache.
Everyone does things differently but I always work with combining my flavorings when working on some combination flavors. I have none of the diluents it of other products nearby even until I have an idea of what the proportions are to get the taste right. Seems like once I can get that then when adding in things like the diluents- some of which have taste you can adjust the mix for the flavor..
My biggest issue with any things that needed a banana and cream mix was that banana extract comes in a wide range and even if you stick with natural and organic flavors and away from artificial it can run from strong to weak, and some with a hint of banana smell, some without and a wide range of 'bannanieness'. Then you want that 'creamy' aspect that can be hard to nail even with using additions like milk and vanilla flavorings to a thick cream flavoring.
I know DYI has the challenge of taking a bunch of single flavors to mix up to a complex flavor without using a pr-mix of one or two others to get the right taste.
My banana cream had the base of banana ice cream flavoring instead just banana extract and needed a touch of caramel and a little bit of citric acid to zero in on what I think banana cream pie tastes like. I do use a sweetener that I get from MBV.