Hi rkk5000,
A softer foam topper would likely be a better choice than a synthetic fiber topper on a memory foam mattress. A fiber topper will only provide some cushioning under the pressure points of the body but it will provide very little primary support (under the heavier parts of the body) or secondary support (under the more recessed parts of the body such as the waist or the small of the back) and won’t redistribute weight along the body surface as well as a foam material. There is more about primary and secondary support in post #4 here. A topper can also affect the amount of body heat that reaches the memory foam which and can affect how it contours to your body which can make the memory foam underneath it feel firmer.
Your insight mattress also isn’t a “total memory foam mattress” and only has 2.75" of gel memory foam in the comfort layers and then has a 1.5 lb polyfoam support core underneath this. Memory foam is only used for comfort or transition layers but is too soft to be used as a support layer.
Unfortunately you are in a difficult position and somewhat stuck in between a “rock and a hard place” when you need to exchange a major brand mattress for another one when none of the mattress carried by the store you are limited to would be a particularly good choice.
There are many other forum members that have been in a similar position and the information in post #2 here includes some “strategies” than can help you make the best out of a difficult position.
There is also more about the most important parts of the “value” of a mattress purchase (or exchange) in post #13 here but the most important factor is always how well a mattress matches your specific needs and preferences in terms of PPP and the most reliable way to know this is your own personal testing or sleeping experience on either a mattress or a mattress/topper combination (see mattress firmness/comfort levels in post #2 here).
The quality/durability of the materials and components in a mattress is number two and I can certainly help you assess this so you can identify any possible weak links in a mattress and make more meaningful comparisons with other mattresses. Post #4 here also has some guidelines for foam quality and density and that can help identify any weak links in a mattress.
Assuming that the specs include all the materials and are correct … the top 2" of this mattress uses low quality materials that by themselves would be “on the edge” of being a weak link in the mattress (more than “about an inch or so” of lower density materials). The memory foam underneath it is also slightly lower than the minimum density for memory foam that I would normally suggest (4 lbs) although it’s only just outside the range. Overall this would be a somewhat risky choice in terms of durability because there are 3" of lower density materials in the top layers of the mattress.
This one would be worse because it has 2.5" of low quality/density foam in the top layer which is also softer and this would be an obvious weak link even without the 3.6 lb gel memory foam underneath it which is also “on the edge”. This mattress would be even more risky in terms of durability than the first one you listed.
You may be misreading the specs or confusing memory foam specs with polyfoam specs but there is no 1.8 lb or higher polyfoam in either of these mattresses and there is also no HR polyfoam in them either (you won’t generally find either of these materials in the comfort layers of the major brands).
Your Insight mattress uses higher quality comfort layers than both of these mattresses (2.75" of @ 4 lb gel memory foam). If there are no mattresses there that are less risky or use thinner layers of lower quality materials in the comfort layers then I would still consider adding a topper to your current mattress to provide the additional pressure relief that you need because even though it can involve some trial and error or the need to return a topper if you make a mistake … by choosing a supplier with a good return or exchange policy you would probably end up with a better mattress/topper combination than choosing a mattress there that would be both suitable for you and used durable materials with no obvious weak links … and it would probably end up being less costly as well.
I know you mentioned that you’ve read them … just in case you haven’t or have read some other topper posts … if you do decide to use a topper then post #2 here and the topper guidelines it links to has more information and sources that can help you make the most suitable and least risky topper choice.
Phoenix