HI Ari,
I’m not sure what you mean by “sleeping on a slab”. Higher density does not equate to “harder” foam in polyfoam.
Every brand has their target demographic - some choose to make good quality products in a budget range, others premium, some less durable. While it is true that most of the larger brands tend to use lower quality materials, not all do, and there certainly are companies producing higher-quality memory foam mattresses. Post #12 here has some of the better online memory foam options of which I am aware.
Realize that a 1.8 lb poly foam core of a higher ILD can still be a quite durable material.
Phoenix