Hi stickhandle,
I’m not sure what you mean by a DIY mattress but if you are thinking about buying separate layers and putting them together yourself I wouldn’t suggest it unless you already have the knowledge and experience to design your own mattress and know exactly what you want. As you can read in post #15 here … working with a knowledgeable manufacturer that makes and sells component mattresses will let you take advantage of their knowledge and experience as well as their exchange and return policies and can often end up as a much less costly and more effective way to go (in terms of cost as well as sleep quality).
The advantage or having more layers is that they allow you to re-arrange them in different ways to customize PPP (Pressure relief, Posture and alignment, and Personal preferences) in more ways. The only thing I would tend to avoid is having a soft layer on the very bottom and other than that … medium under firm is a valid layering option. It will create firmer support layers under the comfort layer than medium over firm which can reduce the amount your pelvic girdle sinks down in the mattress and can improve alignment for those who need a little more support under the heavier areas of their body.
As you can read in post #2 here … multiple layers allow you more options to adjust the compression modulus of a mattress (the rate that a mattress gets firmer with deeper compression) which changes the balance between pressure relief and support. A medium / firm combination for example will become firmer with deeper compression faster than a single medium layer and not as fast as a single firm layer. This would slightly reduce pressure relief and slightly increase support compared to a single firm layer and would act the other way around compared to a single medium layer. The layers closer to the surface will have the biggest effect on what you feel and pressure relief and the deeper layers will have the greatest effect on support but all the layers will act together and every layer will affect every other layer to differing degrees. The middle layer can also act as a transition layer between the pressure relieving layers and the firmer support layers and can have a dual effect on both of the two main functions of a mattress.
The bottom line is that they provide a greater range of customizing options and fine tuning.
Phoenix