Hi debating,
While I don’t know the specifics of your platform bed … it’s very unlikely that it would affect the feel or firmness of your mattress and it’s much more likely that the upper layers of your mattress are firmer than you need or prefer.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper(s) combination is a good “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to 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. It also includes a link to a list of some of the better online sources for toppers I’m aware of.
A good exchange/return policy can reduce the risk of an online topper purchase so if you aren’t certain whether a topper will work well for you I would always make sure you are comfortable with the options you have available after a purchase just in case the topper you choose doesn’t turn out as well as you hoped for because the the only way to know for certain whether a mattress/topper combination will be a good “match” for you will be based on your own careful testing on the combination or your own personal experience when you sleep on it.
Latex and memory foam are completely different materials with very different properties and the choice between them would be more of a preference and budget choice than a “better/worse” choice. There is more about the general differences between memory foam and latex in post #2 here that can help you decide between them but again the best way to know which type of topper material you tend to prefer will be based on your own careful testing or your own personal experience.
“Support” is often misunderstood because the goal of a “supportive” mattress is to keep the spine and joints in good alignment and this requires the type of contouring support that allows some parts of the body to sink in more (softer) and some parts of the body to sink in less (firmer) and this will vary on an individual basis. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support/alignment” and “comfort/pressure relief” and “feel” and how they interact together.
There is more information about wool toppers in post #8 here and the posts it links to but in very general terms … wool toppers will tend to be firmer than softer foam toppers.
Phoenix