Hi zexpress,
If you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources, then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen). I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or alternatively use a “bottom up” approach (see post #2 here).
If you decide to go with the configuration you mentioned, you may wish to consider a Talalay layer under the memory foam, as you have been sensitive in the past to the firmness (bending-in) of the transition layers, and the Talalay will firm up a little less than the Dunlop.
Zoning systems can be useful for different applications (not just firmer support in the middle), such as people that have more difficulty finding a mattress with the right “balance” between comfort/pressure relief (under the shoulders especially) and support/alignment (under the hips/pelvis especially) or who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article and in post #11 here and the additional posts it links to. A zoned or un-zoned pocketed spring unit should both provide adequate support for you.
Phoenix