Thin is a recipe for failure. Industry standards call for it to be something like 1-1/4" thick, but many people seem to get by with it a bit thinner, but not 1/2-3/4". The only time you can successfully achieve reliability is with a bonded mudbed over a slab...not a floating layer over a waterproof liner. Adding a polymer will make the stuff sticky, and MUCH harder to shape. Plus, that layer MUST be porous to allow moisture to percolate through it to the weep holes in the drain...adding a polymer will close off most of those pores, and keep the tile wet. While the tile and grout will slow moisture getting into that setting bed, it by no means stops it...some does get in, then needs to either flow out, or evaporate out (harder to do when it is really humid).
If you haven't done it yet, plug the drain and do a flood test to make sure you haven't compromised things. Also, one common mistake people make is screwing cement board onto the curb...that is also a recipe for disaster. The curb must be covered with lath and mortar. You cannot make any penetrations to the liner below 2" above the top of the curb, and that obviously, includes the curb itself (but way too many people seem to ignore or misunderstand what should be a simple rule).
You can make your mud a bit richer, but I wouldn't get it less than 4:1.
Your climate is an ideal location for a surface applied membrane underneath the tile, but that requires a different type of drain assembly. That way, there's essentially nothing beneath the tile that can absorb moisture, and the whole thing will dry out much faster and more reliably. Hot, humid conditions can make a conventional tiled shower problematic. Mildew requires three things: the spores (everywhere), moisture, and food (soap scum, oils, body skin cells, dust, etc.). The only thing that you can enhance is limiting the water/moisture, which is what a surface applied membrane will do.
