Hi jkozlow3,
Unfortunately … the softness/firmness of a mattress is very subjective and relative to each person’s body type, sleeping style, and perceptions and the specs you listed for the Beautyrest Black Ansleigh Plush don’t provide any “comfort” specs (or quality specs either for that matter) and even if they did it wouldn’t be possible to guess how it may feel for any specific person. Regardless of this though … the extra 1 1/2" of soft foam in the Brooklyn Plush Pillowtop (assuming that it’s the only difference which isn’t clear because they don’t provide any specifics about the density or ILD of the foams) would certainly account for the extra softness.
Even with this … you won’t be able to add the same extra foam in between the quilt and the comfort layers and if they are added on top of the mattress as a topper it would feel different from using the same materials inside the mattress in the same layering order.
Because of all the complexities and unknowns involved … the best way to choose a topper is based on your own experience on a mattress along with the topper guidelines in post #2 here and in post #8 here.
I’m not quite clear on what you mean here because 6" - 8" below the hip bone would be in the middle of your thigh where there is no joint. Perhaps you mean the distance between the iliac crest which is on top of the pelvis and the actual hip joint which is where your butt meets your leg (see the diagram here). In any case … if the upper layers of your sleeping system are too thick/soft (including your topper) then the hips can certainly be flexing out of their neutral position which can lead to discomfort and pain.
The 5.3 lb memory foam you are using is firmer than some people prefer (especially if they are in a lighter weight range) and would also be firmer than many 4 lb memory foams. It can also vary with temperature, humidity, and the length of time you sleep in one position.
Based on your feedback … it would probably be worth testing a 2" layer of 4 lb memory foam and even though it may not be a duplicate of the Brooklyn Plush … it would probably have reasonable odds of working well for you if your experience indicates that “a little to a fair bit” of extra softness would work well for you.
Phoenix