Hi Mish87,
Congratulations on the (near) completion of your DIY bed! Sounds like you found a good arrangement for the comfort portion, and just need a few finishing touches.
The foundation of a sleep system – whether innerspring, platform, or slats – is basically to support the weight of the mattress itself. It should not affect the comfort level of the mattress. A ½" of plywood is relatively sturdy, but of course the thicker the better, as any cracks or splintering could damage the layers which rests on it, and of course, as long as it will support the weight of the mattress, it should be fine.
I would caution you however that you should contact the manufacturer of your DIY layers first, as there are often specific foundation requirements in order to not void any warranty/exchange policy they have in place, just to be on the safe side, since it sounds like you are still ‘sleep testing’ the comfort layers to make sure the firmness is comfortable for you. There should be little to no difference in the feel of the mattress between the plywood board and slats, though the slats ‘flex’ somewhat to accommodate the weight of the mattress, this is structural, not comfort based.
Also, for the platform you plan on buying in the next few months, There are some guidelines for choosing a mattress foundation or platform bed that is suitable for an “all latex” mattress (or latex hybrids or other types of mattresses as well) in the foundation post here, which also includes links to several sources for slatted platform beds that may be worth considering although of course there are thousands of stores that sell platform beds so the list certainly isn’t a complete one.
Hope this helps,
~ Basilio