Yeah, I have done all ultimate weapon sidequests. I don't think you can even get to the Dream Devourer without doing them.
I thought that the way to get to the Dream Devourer was to beat all the Dimensional Vortexes.
Now I've started a New Game +. I just want the "Kill Lavos Early" endings. Breezing through pretty fast. I literally killed Yakra with one regular attack from Lucca.
Wow.