Hi PBFG,
The tutorial post includes a link about reviews in general and they are among the worst ways to choose a mattress (see post #13 here.
You can also read more about BIAB in post #2 here. I would be very cautious about buying any memory foam mattress that didn’t meet the memory foam guidelines in post #10 here in terms of material density/quality.
The thickness of a mattress has little to nothing to do with how well it matches your needs and preferences in terms of PPP. The design of the mattress and the specific combination of materials and components inside it are what is important. Some online memory foam retailers and manufacturers in the memory foam list use some of the more commonly available memory foam mattresses (such as Tempurpedic, iComfort, or Optimum) as reference points so you can test a local mattress that is similar to the one you are considering before purchasing one of their mattresses.
Phoenix