FWIW, I would go soft vintage - but someone’s that overlaps with a more Brit trendy name like Marlow(e).
I would probably not go an A name if you’re intending on more kids and don’t want to continue the pattern for all 4+ - but would be OK doing one if this is your last child.
I wouldn’t go Cassidy or Cassia - don’t think a Cass/Cassie name suits your set. The rest would work really well!
I love Marnie - you could even use Marian or Marianne nn Marnie for something even more genteel.