Hi Dahlia,
You’ve probably seen this but just in case … post #2 here and the posts it links to have topper suggestions and links to the better sources I’m aware of.
You’re most likely correct on both counts (your shoulders aren’t sinking in far enough and this can also lead to leaning your body forward to help relieve pressure).
While toppers can involve some trial and error … if I had to guess given your feedback about the 3" topper and your other comments I’d probably go with 2" and hopefully from a source where you are able to return it. 24 ILD Talalay may also be worth trying because it would allow your shoulders to sink in a little more easily than a similar ILD of Dunlop and is slightly firmer than the N2 (and your experience on the N2 may not have been a reliable indicator if you tested it in the first week or two of having your mattress). If they have Talalay toppers at Bedrooms and more it may be worth trying on the Aspen.
Brooklyn Bedding also allows returns on their Talalay toppers (less shipping costs).
Phoenix