Hi blantonator,
They do have several different types of memory foam (including gel memory foam) that each have their own feel and characteristics so there would still be some options with memory foam although memory foam is a little more tricky because of the differences between different types (see post #9 here) and because they change their response with temperature, humidity, time (becoming softer with continuous compression), and compression.
Ditto to dn’s great comments as well.
You can also add thinner layers of either the same or other types of foam either below or above the memory foam to fine tune it either for softness/firmness or for “feel”. For example a thin layer of latex on top of memory foam has a little more surface resilience but still has much of the “feel” of the memory foam below it. You can also add a layer below the memory foam as a transition layer between the firmer latex core and the memory foam. All of this is part of the “art and science” of mattress design although when it comes to feel it’s mostly art.
I think a 3" layer would be a good place to start and you would also have a few thinner layers that you could re-use (if they are in good condition) for some fine tuning as well.
I would start by testing one layer at a time and then make any changes based on your actual experience with each combination and based on your target for how you want to change each combination.
Some additional sources I’m aware of for covers of all types are listed in the DIY post #4 here.
Good luck … and I’m looking forward to hearing about your experiences and feedback along the way if you decide to go in this direction and of course any questions you have are always welcome.
Phoenix