Hi gardenguy,
Welcome to the Mattress Forum! 
I’m glad that the information here has been useful to you.
Regarding beginning a new DIY project, 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 understand the issue you’re having with the 5" of memory foam on top of your polyfoam core. The latex combination you’re considering certainly would provide more surface support, but still should provide good contouring while you sleep upon your side, and hopefully sleep “cooler” for you. There’s no reason that what you’re considering wouldn’t work for you and what you describe, but as you’re aware this is all quite subjective with a DIY mattress and there can be considerable trial and error, and unfortunately I can’t predict how this might feel for you.
Even though they aren’t a member of the site, I would consider Foamorder to be a reliable supplier. Sleep on Latex, a site member here (which means I think highly of them) would also be a good source of latex componentry and I wouldn’t hesitate to contact them directly via phone for their advice on how their Dunlop latex might work with your particular situation, as I do trust in their product knowledge.
I’m hoping that whatever configuration you eventually decide to choose that it will work well for you. If you have more specific questions, I’ll do my best to answer them as best I can.
Phoenix