Hi tesseraye,
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 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 and 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. 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 just in case the topper you choose doesn’t turn out as well as you hoped for.
Your mattress would be different from most hotel mattresses and sounds like it’s probably firmer so adding a featherbed to your mattress wouldn’t feel the same as adding the same featherbed to a hotel mattress that is different from yours. As you mentioned you would be much more likely to feel the firmness of the mattress underneath a featherbed (or the down alternative fiberbed which you are using) than you would with a foam topper.
The choice between different types of topper materials would be more of a preference and budget choice rather than a “better/worse” choice and since you are the only one that can feel what you feel on a topper or know which type of material you tend to prefer there are too many unknowns, variables, and personal preferences involved in choosing a topper to make any specific suggestions.
If I was in your shoes though and only needed a topper for a year … and assuming that your mattress is in very good condition without any soft spots or sagging … I would probably choose a softer polyfoam topper or possibly a memory foam topper if you prefer the slow response of memory foam vs the more resilient response of polyfoam (this would be a preference choice). A good quality polyfoam topper would be a much more economical choice than a good quality memory foam topper as well. If you are only using a mattress for a year then it may not be worth considering a latex topper which tends to be the most costly foam material unless you don’t mind spending the extra money for only short term use.
I don’t know the thickness of your mattress but there shouldn’t be an issue with buying sheets as long as the pocket depth of the sheets are suitable for the height of your mattress/topper combination.
Phoenix