You've got it, I think. If you give the verbal and hand signal simultaneously, a lot of dogs will either learn that the combination is the cue, or that the hand signal is the primary and most important part of the cue (and maybe ignore the verbal entirely), because dogs are hardwired to pay significantly more attention to body language than verbal communication. It can take some pretty conscious effort to get the timing down, but definitely try breaking it down into verbal>hand signal>marker>reward, completing each step before starting the next. I know I've accidentally worked in treats to some of my cues by reaching for the reward as I was cuing, and had to go back and be more conscious of my timing to break that association.
Some dogs are more forgiving and pick up on what you want even if your timing's mushy, but even they tend to respond better to good timing because it makes communication as clear as possible.
