Leverage the Drengin's military advantage. You should have been picking military production bonuses already as Drengin are all about conquest. But you cannot grow and support a strong military without a strong empire. Boost military production enormously from the racial picks - and colony rush. Now as you see, the military bonus naturally applies to the colony rush. Requires a balancing act with economy, but burn much of the initial money fast.
It is much easier to colonize than it is to conquer. Difficult or not as it may be for you to conquer others, it will be absurdly so for them to try to conquer you in the early years, as not only do you have the best troops but they probably do not even have planetary invasion and have no way to attack the worlds which you have seized out from under them! So get those worlds fast and you do not have to worry over them.
Once you have a vast empire you are set. Colonies do not have to be developed for your empire to be considered mature; an "empire" of outposts with a developed core is sufficient to commence galactic conquest.