Love your girls names!, all except Iris. Which is ok as a middle name.
Susannah - lovely, Susie is gorgeous Joy - lovely, could never understood why Grace wsa so ubiquitous and Joy completely overlooked. Love Joy. Iona - love this one! Phoebe - LOVE this Louisa - Love it.
Eliza, lovely but mighht turn into another ellie, that is what would put me off using it as 'the one'.
From your boys names, I love Rory and Nathaniel the best. Rowan and Tristan are nmsaa.
If so, I'd pick a couple at random and start using them to see how they stick.
If I had free choice from your lists (i.e. no dh with pesky opinions!) I'd go for Nathaniel for a boy and Rowan or Susannah for a girl. I'd use family names of some sort as second names as I've always liked families where this happens.