Hi Sepia,
Sorry to take so long to reply to this … I was up till about 5:00 AM last nigh/this morning and for some reason I skipped over your post (and a couple of others).
This would depend on many factors including your body shape, overall health condition (in terms of muscle tone etc), and sensitivity to pressure as well as the combination of the firmness of the topper and the firmness of the base layers below it.
In general terms a 3" - 4" comfort layer (what is in the mattress along with ny topper combined) would be in the average range and since your comfort layer is quite soft you may need something on the thicker end of this range to help isolate you from the firmness of the base layer.
I would use the topper guidelines in post #8 here along with post #4 here to help you decide how much extra thickness/softness you may need.
This is really a matter of personal preference. If you prefer the feel and performance of latex vs memory foam (the two most common topper materials) then I would go in this direction. With a mix of the two materials … you would have a combination of the feel and performance of both which some may like and others not so much.
Another option if you want to experiment a bit with a “cheap” topper (to see if it moves you in the right direction) before deciding on a higher quality more permanent replacement is in post #16 here (this is polyfoam which is a “fast response” material).
I would tend towards fine tuning what I had because its a clear reference point (and to save the expense and hassle of a return) if that was possible but this is also a personal decision of course and would depend on how you feel about one option vs the other. Keep in mind that if you order a mattress with a different combination of materials that it may also need some fine tuning because unless you have actually tested it … you won’t know for certain that the specific combination of layers is what you need and prefer.
Hope this helps.
Phoenix