While it’s true that you could have bought another mattress … at least you have the comfort of knowing that what you did buy is good quality.
Your comments about the Tempurpedic Grand being too firm are interesting and they go to show why memory foam can be quite tricky when it comes to softness/firmness. The Grand is the softest mattress that they make outside of the Cloud series and they rate it the same as the Cloud itself in terms of “feel” (medium soft) although with the HD memory foam it is also more highly conforming which generally translates into more pressure relief. with memory foam though … particularly higher density memory foam … it can take longer to respond so the firmness that some people can feel is not so much about the softness itself (all memory foam is soft based on ILD which is a softness rating used for foam) but about the length of time it takes to reach it’s softness level with movement and how it “resists” changes in position. Even wood can be very pressure relieving (evenly distribute pressure along its surface) if it is carved to the exact shape of the body and someone doesn’t move at all. As soon as you move though … its firmness becomes evident and of course it doesn’t adapt like memory foam. Memory foam in some ways is like a liquid (the viscous part) and like a thick liquid … it can be soft if you press into it slowly but very firm if you press into it or “slap” it quickly.
So this leads to the type of topper you would prefer. There is really no better or worse between different materials like latex and memory foam in terms of how they feel and it’s really a matter of preference which one is best for each individual.
Softer less dense memory foam tends to be less temperature sensitive and a little faster responding so if you are looking for the type of softness that equates to faster response … then a topper with say a 4 lb memory foam that is faster responding and less temperature sensitive (responds similarly in a wider temperature range) may be the way to go.
On the other hand … if you want a faster responding soft topper that takes on the shape of your body more quickly and doesn’t “hold you in position” as much … then a soft lates would be the way to go.
Either way … it will have an effect on the performance of the memory foam below it because there won’t be as much heat that reaches it which will slow down it’s response. My personal preference would be latex but that’s only because I much prefer a faster responding foam and I also like the more “lively” feel of latex with some of the “give” of memory foam below it. Again though … each person’s preference may be different.
Other options would include a polyfoam topper (also more instant responding but not as durable, elastic, supportive, or conforming and pressure relieving as latex). A wool topper is also an option although this will also firm up the memory foam below it because it will prevent as much heat from reaching the foam. It is highly breathable so will help with temperature regulation but wool will also compress over time and become firmer. It provides more localized cushioning for the more protuding parts of the body rather than providing pressure relief over the entire surface of the body like a foam. My guess is that it may not be the best choice if you are looking for more overall softness.
The topper choice of thickness and firmness will also depend on how close or far away from your ideal you are in terms of pressure relief, alignment, and “feel” and also on your height, weight, body shape, and sleeping positions. In general … “just enough” additional softness is much better than too much because this will have less of an effect on your alignment while you are sleeping.
Overall … I wouldn’t go over 2" with a mattress that already had this much memory foam and the thinner/firmer the better. “Just enough” is always the best. With a memory foam topper … because you already have a thick layer of memory foam in the mattress … I would again use the thinnest layer you could “get away with”. With thick layers of memory foam (topper plus what is already there) … you can easily sink too far with the heavier parts of the body over the course of the night (memory foam “relaxes” or “creeps” with constant pressure over the course of the night) and thicker layers of memory foam can be quite risky in terms of spinal alignment even if it is relatively firm (compared to other memory foams) or high density.
I would also choose latex over polyfoam unless price was a main consideration in which case I would use the highest quality polyfoam I could find. This would be a good example of a high quality polyfoam (note the description mentions memory foam but this is polyfoam that has shape conforming properties like memory foam and latex but is not slow response).
So hopefully this can give you some guidelines that can make your choice and some of the tradeoffs you face easier and help you get closer to the “ideal” you are looking for.