I have reached an epiphany with the Fallen Hope. I have figured out how to make it work.

In order to do more damage, I must do less damage.

In the place of my damage mods, I must rely on the bonus against statused targets.
And I must max weak point, so that I can choose between high damage when hitting the head of elites, or low enough damage to not instantly kill the mooks.

Now my poison explosions are actually working, every tenth mook is getting statused, and every third statused mook starts a chain reaction. I've now gone from seeing explosions maybe twice a dungeon to multiple times a room.

they're still shit though