Hi Tizzy,
Have you talked with Novosbed and did they make any suggestions?
Based on your comments and their reply in this thread … it seems that they also think that the mattress you have is too firm and you may be better off with something softer.
If you do decide to go with a topper then the topper guidelines in post #2 here and post #8 here may be useful.
If your local testing on the mattresses that “match” the ones they sell and a more detailed conversation with them gave you good confidence that a different mattress would be a more suitable choice then I would personally lean towards a mattress exchange rather than the greater uncertainty (and cost) of adding a topper.
Phoenix