"One thing I've held onto from Lugia's redux but I'm not totally convinced I need is the expander unit for the Zadar. I'd love to hear from people who use it about why they do or do not feel like the expander is essential."
It's just an extra set of CV inputs. How essential it is to you will depend on how you like to patch. I can't have enough modulation. It doesn't have to be in your face. You can add tons of subtle modulation of the kind you feel more than hear. So for me personally not only it is essential, I wish I could daisy-chain more...
The trigger buttons are useful for testing stuff quickly (and I guess for live use, not my case) but definitely not essential to me.