Hi wildflower,
This is one of those issues where you aren’t going to find consensus among manufacturers and for those who are used to finding one single “right answer” this can be frustrating because there isn’t one in my experience. You can read more of my own thoughts in post #10 here. In a dry climate mold and mildew would certainly be a much lower risk.
The issue of a solid non flexing surface and flexible slats is the same. Conventional wisdom says that a solid non flexing slatted surface is “best” but there are also manufacturers who are adamant that a flexible slat system or a box spring are the best and their customers seem to do well with them for the most part.
Since you already have the flexible slats … I would start with them and let your own personal experience on the combination in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) be your guide as to what is best for you. It won’t harm the mattress so it’s really a matter of how well it works for you.
Phoenix