Hi waterbugs,
My first suggestion would be to talk with Shawn at SleepEz because they are happy to provide ongoing guidance to any of their customers and if you do decide on a topper they also provide a discount to their customers off their regular topper prices. If you do go in this direction I would probably keep it to 2" of “soft” because you probably only need a little bit of extra softness/thickness on top and you would want to make sure that you don’t have too much thickness for your hips to sink down into the mattress too far before reaching the firmer layers which could risk alignment. They would have many customers that they have helped in similar circumstances so it would make sense to talk with them and see what they suggest.
I think the Seven comforts topper would also make a good choice. It’s advantage is that it is shredded latex (microrods) which means it can both displace and compress in different areas. It will allow the shoulders to sink in a bit more (displacing some of the latex) while still supporting the broader hips/pelvis (less displacement and more compression) which means it can add pressure relief without compromising alignment. Of course your personal experience may be different but in “theory” it should work well.
Edit: see post #52 here first if you are considering ordering this topper.
Phoenix