Hi Sleepymucho.
As Pure Green medium was too firm for both of you and adding the 2" soft topper proved to be good for your partner but too soft for you, I agree that probably the 1" topper may be the best solution for both of you to have the “middle ground” softness/firmness combination. As your partner is primarily a prone sleeper I would be hesitant to suggest the softer version of SOL as this may cause sleeping in a swayback position that might result in lower back pains. There’s a bit more information about common sleeping positions and appropriate sleeps surfaces here.
If both you and your partner decide to switch to a spring mattress, I would pay the most attention to your own careful and objective testing which will tell you how the innerspring performs in combination with the other layers and materials in a mattress (versus using a latex core) and to the quality of the materials above the innerspring which is normally the weakest link in a mattress. You certainly can find a comfortable combination using springs or latex. There is more about the different types of innersprings in this article , in post #10 here and in post #16 here
and you can use your previous experience and testing and seek guidance from some of our Expert and Trusted Members that carry innerspring hybrids and can use the information you provide them with about your body type and sleeping positions, your preferences, and the history and experience with the previous mattress to recommend something that would be suitable for both you and your partner.
Hope that the new mattress/toper combo will work out for you and I look forward to any updates you might have along the way
Phoenix