Two thoughts:
Not all LED strings dim well, but if they are really full wave, you should not have a lot of flickering.
Neither the dmx SSR4 or the freestyle/dumb SSR4 combo have support for light curves like the express. That means you are stuck with the standard curve, which is likely just linear... LEDs will not turn on until you reach the threshold voltage and from there will dim up to full bright rather quickly with the last block of values being nearly indistinguishable. It's just the nature of the device...