Hi nick31.
Welcome to our Mattress Forum. 
Indents from bodies mean premature breakdown of materials (usually). Of course, it can also occur with the natural lifespan of a mattress, but 6 years is not especially long. If you haven’t already, I’d suggest having a look at our Durability Guidelines.
Sleeping hot can be to a number of factor. Phoenix has a super detailed post here about the various things at play in mattress microclimates that you may find interesting.
Without knowing the exact specifications of the other mattresses, it’s hard to make a comparison. Ultimately, even changing 1 small thing (from the encasement to the foam) can change the feel. So, instead of trying to approximate the feel of other mattresses, we recommend just aiming towards what feels best and most supportive for you.
At first glance, your proposed build looks great. You both prefer firm. Dunlop tends to “press back” a little more than Talalay (which is lighter and springier). I’d suggest the 4" broken up into 2" slabs for a little extra cushion in the support/transition layer, but also for easier layer swapping, should needs arise.
NikkiTMU