Type and Thickness of Topper?

Hi. So, I have an pretty firm bed; bonnel springs. It’s the hardest I’ve ever slept on and need to soften it up a bit. I’m a 5’5 female, 135 lbs. I’ve had an injury to my upper back years ago which resulted in some instability in my thoracic spine. All that means, is that I must have a firm base to keep my back aligned properly. But this bed is so hard that sometimes both my arms go numb…ugh. So, I need a topper to soften it. With my weight would it be better for me to get a ‘soft’ latex topper or a medium one? and how thick would I need? I mean, I can try them in the store, but they probably wouldn’t have the same mattress I do. So, it would not feel quite the same. I’ve never had a latex topper so I sure hope I don’t have any allergies to it, but I tend to sleep warm sometimes so I hesitate to try a memory foam one. Oh, and I would say I sleep about half on my back and half on my side. Thank you for your help.