Hi diamondshopper,
You already have some experience using a softer layer of latex (you didn’t state if it was Dunlop or Talalay), but this would have been in substitution of a layer of your current mattress as opposed to on top of the product, so you can relate a bit of what you experienced to getting a new topper, but it will be a slightly different experience upon your current mattress.
As you tried out 3" of a plush layer and it allowed you to sink in too much, I think you are on a good path considering a 1" or 2" topper. If the mattress needs just a “touch to a little” extra softness or a softer “surface feel”, then a 1" topper is usually enough. If you need a little to a fair bit of extra softness and pressure relief, then a 2" topper would be in the average range.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to in post #8 here which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
The topper and component reference post here includes the better online sources I’m aware of for latex toppers and components. Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person, a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesn’t turn out as well as you hoped for. While not a complete list, here are some toppers that can be returned of which I am aware.
Phoenix