Hey there Spincycle,
Wow, so you have a pillow menagerie like me. Except I still have every pillow i have tried, all over the place in every bedroom in the house.
Although it is easier to trial and error through a bunch of pillows than it is a group of mattresses, it is not uncommon or unheard of to trial a few mattresses.
Diamond actually makes some nice mattresses in the USA. I am not a big advocate of mineral infused foams. Mattress manufactures have stated that over time, the mineral/copper infusions pull the moisture from the foam and causes the foam to crumble over time. So be cautious with any infused foams.
Speaking with another sleeper who was having similar issues as you, a solution was found with a wool topper with a latex core. Something like this one or this one and even this one
It is a bit pricey but there are some options out there. The natural sleep store offers a couple of toppers, one by harvest, that has a latex core with a cotton or wool cover and a few others, such as the wooly topper that offers a wool and microcoil combination. There is even a horsehair and wool/cashmere topper that a fellow MUGster purchased and said it is just great.
If you are generally happy with your mattress, focusing more by adding a great topper may be the better solution.
The DIY with 3" of memory foam, you may find hot, so again use caution when doing a DIY. You would be better served collaborating with a Trusted Member here, like @Arizona_Premium or @Latex_Mattress_Fact1 to narrow down the best combination as they are sharp shooters when it comes to successfully helping folks DIY.
Often, taking a step back and looking at your situation from a different view can help you think this through a bit better.
Good luck with it,
Maverick.