Hi goldilox,
Unfortunately there are too many unknowns, variables, and individual sensitivities and preferences involved and I can’t feel what you feel on your mattress so there isn’t a formula or “theory at a distance” based on specs (either yours or a mattress) that can predict which topper will work best for you with any accuracy. My “best” suggestions are in the topper guidelines and the posts it links to which will allow you to make a “best judgement” choice based on your own actual experiences. The information about “how” to choose a topper is fairly extensive and any additional guidelines I could add here would only be repeating the information that is already there.
The topper you have is 15 ILD which is very soft and that along with the type of symptoms you are experiencing (lower back ache) and the improvement you noticed when you changed from 3" to 2" “points to” the softer upper layers of your sleeping system (mattress and topper) being too soft and/or thick for you.
It seems to me that using a slightly firmer topper or a slightly thinner topper (or both) would have a greater chance of finding the best “balance” between the often conflicting needs of comfort/pressure relief and support/alignment. In general it’s usually a good idea to have “just enough” thickness/softness in the upper layers of your sleeping system to provide the pressure relief you need in your most pressure prone position (in your case that would be your side) so that the risk of alignment issues and discomfort or pain in your lower back when you sleep on your back is minimized.
The type of material you choose is a personal preference so you would need to decide whether you prefer the feel of memory foam or latex or polyfoam or any other material you are considering or whether you prefer the feel of Dunlop or Talalay. Post #7 here has more about the difference between Dunlop and Talalay and post #2 here has more about memory foam vs latex but your own personal experience is really the best way to know which type of material you tend to prefer. A wool topper will tend to be firmer than soft foam but a shredded latex topper may also be worth considering because it can add some additional softness with less risk of alignment issues.
Latex International no longer makes the Talalay GL slow response because it wasn’t popular and most people that preferred a slow response material ended up choosing memory foam. You may be able to order it though until supplies run out at some of the sources that are listed in the component sources post (KTT Enterprises may still have some) but you would only be able to exchange it for another topper that Latex International makes (Talalay latex) because they only have an exchange policy not a return policy.
The first step though would be to sleep directly on your mattress for a few days or for long enough that you can decide on “how much” additional pressure relief you need (a “touch to a little” or a “little to a fair bit” or a “fair bit to a lot”) and use that as an initial reference point.
You may need to go through some trial and error (just like when you were testing mattresses) to end up with the choice that works best for you so I would make an initial “best judgement” choice but also make sure that the retailer or manufacturer you purchase from has options after your purchase to either return or exchange your topper for a reasonable cost so that the risk and cost of making a mistake is lessened.
Phoenix