Hi TossAndTurn.
I can’t speak to the quality of the brands you shared since I am not familiar with them, but if they appear adequate for your latex mattress and budget, they may be worth the purchase! “Bowing” may occur on the mattress if the foundation doesn’t adequately support its weight or your weight, especially if there are two sleepers with very different BMIs or if the frame isn’'t properly put together.
If you look at our Trusted Members’ sites, you’ll see the recommended foundation for latex has boards all he way across, like this frame from AZ Premium (which appears to be in your budget).
The platform/space saving foundations are interesting. I definitely see the appeal, especially when one has less space than they might like. I wonder if you could create some sort of hybrid with the slatted base to allow for some air flow. All of the slatted wood foundations I’ve seen sit rather close to the floor and aren’t great for storage, but I imagine there is a way to elevate those bases or, as you previously mentioned, maybe making your own base so that it’s to your ideal specifications.
Looking through the Trusted Member websites may be useful for you as far as creating a plan for what you may ultimately choose to build (number of slats, slat width, etc.)
I am sorry I don’t have a fixed solution for you, but hopefully something I have said here can help set you in the right direction for your needs.
NikkiTMU