Hi BRoel.
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for a combination of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress. There are just too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see post #2 here).
While I don’t have the exact specifics on your mattress (ILD and density of the core), an N3 topper (if it’s the Natural Vita Talalay) is very plush, so going softer wouldn’t be an option. From a distance there could be quite a few different things coming into play. Your level of the flexibility of your hip flexors immediately comes to mind. Have you tried sleeping with something under your knees to see if this makes a difference? Additionally, it may be a case where only 2" of the very plush latex doesn’t provide you enough cushioning, allowing your hips to sink through to the polyurethane core. A zoned latex topper will allow for more sinking in your shoulder area, but it will also allow for more sinking in your hip area, which you said was an issue. But how much would depend upon the firmness and thickness of the zoned topper you choose.
As you’ve only had the mattress for a short period of time, see if you can make it through a few more weeks to see if you adjust to the mattress and the mattress “adjusts” to you.
The farther a layer is from your body, the less you will notice the difference. I don’t think placing a layer of poly foam at the bottom of your mattress will do much.
Phoenix