Don't do more than you already need to. WD -> d-tilt you are supposedly already holding down. You don't need to move the stick all the way down, but I would make sure you are more down than side. You are down smashing most likely because you are returning your stick into neutral which you do not have to do. Another example for the sake of examples is shine -> WD. Since shine requires a down input, I would keep the stick downwards instead of going back to neutral before the WD. It is good to practice clean inputs, but also be efficient with it.
Regarding tilts, some have already said, you can practice tilting the stick halfway so you don't trigger a dash or a smash attack. I still press down all the way for d-tilts, though I just don't press A at the same time. If you are in the middle of an action, such as a throw like @
SAUS
said or a jab, you can smash the stick all the way because there will be no action due to you being busy.