Hi Fiddler,
As you already know … the comfort layers of this mattress has 4.5" of polyfoam in the top 6" of the mattress (and some descriptions say it’s more because most of the listed specs don’t equal the thickness of the mattress) which is usually a recipe for foam softening and impressions.
The good news in this case is that the impressions happened earlier and you are approved for a warranty exchange. Not everyone is so fortunate.
While I have no personal experience with this combination, there are some topper guidelines in post #8 here which can give you some insights into the thickness and softness of the topper that may work best for you on a River View which only has an inch of polyfoam (which they call “high performance” which may indicate better quality/density) in the comfort layers. Because you are heavier and a side sleeper I would tend towards a little thicker (3") and a little firmer than “average” but not so firm that your wife who is also a side sleeper would also have pressure issues. 19-24 ILD is generally considered to be in the “soft” range and 28 - 32 ILD is in the “medium” range. The odds say that the lower end of the “medium” range may be your best compromise but of course your own experience, judgement, and intuition is also an important part of your decision.
Hopefully others that have tried this specific combination will also see this and share their experiences.
Phoenix