This is an instance when you can let your ballast grains do the work for you. As described by others, let the flex track sag toward the lowest point if you can't create a proper vertical curve from level and on into the grade. (Many of us use cookie-cutter platforms where a thickness, cut to width for the roadbed, of plywood is screwed flat with the base over four-six inches, and then you screw the far end onto the steady portion of the grade, usually atop a riser cut and positioned to the right height to support the top end of the roadbed.)
The flex will sag, and you can fill part of that gap with door frame shims, and then fill it up to the ties with more ballast grains that normal. Once you glue it, the tracks will be well supported along the vertical curve, just like the prototype.