Thanks, everyone. Some great suggestions, but considering the amount of time I've been obsessing over a middle, really nothing I haven't considered. I think my top four right now are:
Iris OliviaIris FionaIris ElizaIris Adele
I sort of love
Iris Vivienne, but the pronunciation just doesn't feel natural. It's a gorgeous name, but it's like it's trying too hard or something. Maybe if I had French heritage it'd make more sense. Also like
Iris Miranda, but H has vetoed. H loves
Iris Caroline, but I just can't get on board.
Iris Juno is definitely in the right direction, and I think if there was a name like
Juno that was 3 syllables, I'd be sold. I find that J middles don't typically sound the best with our J last name.