Hi Jesse,
There is some “controversy” about this (see post #10 here) but I am one of those who thinks that when possible, slats are a good idea because they allow for more ventilation and control of humidity in the mattress. This humidity and moisture control will reduce the risk of creating a favorable environment for dust mites (which require higher humidity levels) along with mold and mildew. It can also have a minor effect on temperature regulation but this is a less important part of the benefits of a base that can ventilate because sleeping temperature has more to do with the upper parts of your sleeping system closer to the body than the lower parts.
That doesn’t mean that a solid foundation is a “bad” idea and there are some who believe it’s fine and there are some cases where it is unavoidable (such as those that wish to use an adjustable bed that doesn’t have slats underneath) but IMO it is preferable when possible and reduces the risk of unwanted “guests”. A product such as this bed rug 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) can provide good support and can also increase ventilation under the mattress for those who have a solid surface under their mattress.
A solid non flexing slatted foundation will be similar to the floor, a solid surface foundation, or an adjustable bed because all of these provide solid even support that doesn’t flex. The slats should be a maximum of 3" apart and preferably less to make sure that the latex doesn’t sag into the gaps and should have minimal or no flex. Typically this means a minimum of about 14 1 x 3 slats or more for a queen size mattress.
If you are able to return the foundation … then something like the foundation described in post #15 here would be a good choice for a foundation with flexible slats. The original post in the same thread includes many other foundation options.
I think your step by step approach is a good one and hopefully some testing on the correct mattress will give you a good reference point.
Phoenix