Hi moosemom,
I’m glad you found us … and welcome to the forum 
I’m guessing you’ve read it already but just in case you haven’t I would make sure you’ve read the tutorial post here.
For most people 8" to 9" of latex is plenty but it would also depend on the specific design of the mattress and the body weight and sleeping positions of the people on it and which was the best match for them in terms of PPP. At your weights most people would be fine with this or even less than this in some cases. Post #4 here has more about the effect of thicker individual layers or the mattress as a whole.
I would avoid trying to “design” your own mattress based on theory or specs and use your own personal testing to decide on which mattress design works best for you (see mattress firmness/comfort levels in post #2 here). The more generic guidelines can give you some sense of the concepts that are involved in different designs but your body will tell you much more than any “theory at a distance”.
As you mentioned … I would avoid a solid surface unless there was a “compelling reason” to use it (see post #10 here). If the distance between your slats is too much you could either add more slats to your bedframe or if the gaps were just a little bit too wide you could add something like this bed rug here or one of the slat conversions here (which has no flex at all) or even one of the Ikea slatted bed bases here (which has some flex which may change how the mattress feels and performs) to even out the support under your mattress. At the very least you could use pegboard which will allow some airflow under the mattress although not as much as slats.
I’m looking forward to finding out about your experiences and your final choice and of course feel free to post any questions you have along the way.
You have some good options available to you.
Phoenix