Hi pmsmith,
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress and it’s still in good condition then a good quality topper can 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 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 mattress/topper 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 for any specific person on any specific mattress.
There is more information about choosing a topper that also includes a link to the better online sources I’m aware of 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.
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 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 the topper you choose doesn’t turn out as well as you hoped for.
On the other hand … it’s generally much more difficult to “fix” a mattress that was too soft initially or that has developed soft spots or is sagging (which is very likely with a major brand mattress after 6 years) because the best solution would normally be to open up the mattress and remove the layers or components that are too soft or have developed soft spots or are sagging and replace them with new layers. A topper will generally just “follow” any sagging or soft spots underneath it and at best it may be a partial or temporary solution and at worst it can make any issues that you are having with your mattress worse. For example if you add a firmer topper you could end up with a sleeping system where the top layer (the topper) was too firm for you to provide good pressure relief but the layers underneath your topper could either be too soft or sagging too much to provide suitable support to keep your spine in good alignment.
If you have limited alternatives because of budget considerations then there are also some suggestions in post #4 here that may be helpful for a mattress that is too soft or has developed soft spots or is sagging at least on a partial or temporary basis.
I would also make sure that the support system under the mattress is also in very good condition and has a firm, flat, and non yielding surface because if your support system is sagging and no longer provides suitable support for the mattress then this will also cause the mattress on top of it to sag as well.
Phoenix