Hi moboe,
Post #2 here and post #8 here include the topper guidelines I would use.
The thickness and ILD that is most effective will depend on the firmness of the mattress underneath it because all the layers of a sleeping system will interact together (the same topper will feel different on different mattresses). I would tend to follow the more subjective criteria in the guidelines and if you are looking for “a little to a fair bit” more softness then I would go with 2" and if “a fair bit to a lot” is more in the range of your experience on your mattress and the extra softness you think you will need then I would go with 3".
Phoenix