Hi Debbilc21,
It’s always a good idea to talk with the manufacturer of your mattress about any fine tuning you may want to do. This is particularly true if they are knowledgeable which Alan at foam order certainly is. They would have good experience in fine tuning their specific mattresses after a sale based on the feedback of their customers.
I don’t have a reference point of the specifics of your mattress or the Escondido mattress or box spring you liked so it’s not really possible for me to make a specific suggestions because your mattress may be different from the Escondido mattress and you may not be able to find the boxspring they used. While a flexible slat system will provide some extra give under your mattress … it may not be the same as the Escondido combination.
My tendency though would be towards the topper because you may not be able to find the combination that matched the Escondido mattress and box spring and the cost of most flexible foundations or adjustable slatted foundations would not be much different from a firm foundation and a topper and this would be much easier to estimate more accurately. This is also because your mattress may still be too firm even with a flexible foundation or box spring.
A good general suggestion for a topper that would apply to many people with a firm 6" latex mattress would be about 2-3" of softer latex. Your weight may do well with about 19 ILD and I would probably tend towards 2" because of your preference for a firmer mattress.
If you do decide on a flexible slat base or tension adjustable base … there are some sources of tension adjustable slatted foundations in the foundation thread here. This one may be a good choice because it is fairly low cost and can be used as a flexible slatted base under certain parts of the body but can also be used a a solid base if that doesn’t work for you. The Idea options are also low cost but they need a base to go under them (such as an Ikea bed).
I don’t have any specific information outside of what is on the website no.
Phoenix