I've never tasted soylent. But note that in order to make a flavor more powerful, you'd want the flavor ingredient in most quantity and a higher concentration. And the easy way to make any flavor "appear first" is to make it touch the tongue first.
Say, if you mix in lemon juice for flavor, consider spreading finely chopped lime zest on top of the glass. And maybe also throw in the drink something that goes with lemon. Perhaps mint leaves?
Now, as you bring the glass to your mouth you'll first smell citrus. next, the sour-bitter-sweet zest will touch your tongue and hopefully be more to your preference of dominance. Later you will encounter the mints, as if you were casually eating a rarely disgusting Ben & Jerry's.
Of course I don't know if Soylent goes with lemon. But either with chocolate, hot spice or tea (can you infuse tea in it? maybe on a very low flame?) I think the general approach is worth trying.