Hi pinkpantherdee,
Congratulations on your new mattress 
I hope you have the chance to share your thoughts and feedback when you receive it and have had the chance to sleep on it for a bit.
@ Chilliebump.
There can be some additional risks and uncertainty involved in purchasing a topper where you haven’t tried a specific mattress/topper combination in person (see post #2 here) but a topper can also be a good solution for adding additional pressure relief to a mattress that is too firm. If you do decide to go in this direction then then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a guideline to help you choose the type, thickness, and firmness of a topper that would have the best chance of success and includes some of the better sources for toppers that I’m aware of as well.
The 5" and the 10" model are designed to have the same “feel” and firmness level although for some people that are in higher weight ranges and compress the mattress more deeply the thicker mattress may feel a little softer.
This would be a suitable base as long as there aren’t any additional risk factors that could make a solid surface more risky in terms of developing mold or mildew from the more restricted airflow under the mattress (see post #10 here).
Phoenix