Hi rsvoir,
Softness and firmness are very relative and subjective terms and what each person feels as soft or firm can vary widely depending on body type, weight distribution, sleeping positions, and individual perceptions.
There are also different types of softness or firmness that different people may be more or less sensitive to (see post #15 here) and a mattress is always a combination of softer and firmer layers not just a single “rating”. For example a mattress with 2" - 3" of ultra soft foam on top of firm foam would feel quite “firm” for most people because you would likely “go through” the top 2" and feel more of the firmer layers below it. Lighter people would feel more of the top layer while heavier people would feel more of the deeper layer.
It’s usually more effective and accurate to think of a mattress in terms of specific combinations of softer and firmer layers that each perform a different function which includes relieving pressure, keeping the body in alignment in all your sleeping positions, and providing an overall “feel” that is a preference for each person. Each layer plays an independent but interconnected role in a mattress.
“Support” is really just the amount that the firmness level of the foams under each part of the body will hold up weight or “allow” sinking in so that some parts can sink in more than others and the spine is in alignment in all your sleeping positions. As an example … with side sleeping the shoulders need to be “allowed” to sink in enough that the pressure point is relieved and the upper spine is in alignment while the heavier pelvis needs to be “stopped” by the deeper layers before it sinks down too far and tilts the pelvis which puts the lumbar out of alignment. In addition to this there needs to be enough softness under the recessed curves of the body (such as the waist and small of the back) so that the “gaps” are filled in which provides secondary support to the inward curves of the spine. You can read a little more about primary support, secondary support, and pressure relief and how they interact in post #4 here.
The support core of a mattress has only a secondary effect on durability (a mattress will soften and break down from the top down) but at your weight the top part of the deeper layer could be part of your comfort layers or “comfort zone” (the depth you sink into the mattress) which means that its durability would be more important than it would for someone lighter and I would consider a higher density foam under the top layer. You can read more about the factors that can affect durability in post #4 here.
Only each person can decide for themselves whether something is “worth it” because it depends on which of the tradeoffs involved in each person’s personal value equation is more important to them but if it was me it would be worth it both because of the higher density base foam and because it has wool in the quilting (which helps regulate temperature).
Once again whether a choice is worth it is up to each person (all their mattresses are “worth it”) but you can read more of my thoughts about the Alexis vs a latex/polyfoam hybrid that has one comfort layer in post #2 here and if a mattress with more latex was in a comfortable budget range it would certainly be worth it to me.
Phoenix