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Opinions will no doubt abound. My personal bias is paintings under 3 sq. feet are best executed on panels; larger than that, on stretched fabrics.
Extreme changes in humidity (how extreme?) will be problematic in this case, regardless of what materials are employed. On the other hand, you may be overestimating the intensity of this "shock".
High-quality plywood would be my choice in this instance, regardless of how the painting ground is prepared. An oil ground on the wooden panel would be "best" in my opinion, with the panel prepped both sides. It is well to consider the fact that it is impossible to "seal" any wooden item from changes in ambient humidity, even with all surfaces coated. In fact, it is often inadvisable to attempt it.
For my part, it would be better to use plywood 1/2" to 5/8" in thickness rather than thin 5mm (about 1/4"?) stuff. Reinforcing a panel with a cradle is a task demanding a very high level of wood-working skill. The cradle bars need to be fitted very accurately where they cross and interlock, and should not be glued to the panel along their entire length, as this will inevitably "telegraph" to the surface of a thin panel face. Suitable cradles are glued to the panel edges at the tips of the bars, and "float".
A good alternative to a wooden panel with an oil ground would be 1/16" thick ABS (acrylonitrile-butadiene-styrene) sheet laminated to suitable plywood. In the event severe changes in humidity cause the plywood to fail, an ABS veneer could be re-lined with another panel quite easily.
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