It seems common-sensical that the existence of something logically precedes its self-expression.

Trinitarian theology assaults that common sense.  There is no Father except as He has a Son; no Father who has not always already generated His perfect image and likeness; no God who has not always already expressed Himself in His eternal Word.

It is so for everything.  The table across the room doesn’t intend to express itself visually to me, but if it didn’t then I wouldn’t know it was there.  I think it fair to say that a table that completely and entirely failed to express itself would be a not-table.

For humans, there is often a gap between what we are and what we pretend to be.  There is still an unbreakable link between existence and self-expression, though the self-expression is a false image.  That is our fallenness, or (perhaps) our immaturity.  For God, there is perfect, spontaneous correspondence between what He is and how He shows Himself.