I love all the names on your list!
Leonie - late summer on a French estate 
Sylvana - sunny and bright but also a little mysterious 
Isola - sparkles like the ocean 
Apolline - sweet like blossoms in spring 
Catherine - strong and timeless, a great middle name choice 
Cassian - brave and noble like a knight 
Ambrose - cosy like an autumn day at golden hour 
I love the idea of Leonie Catherine 
it feels timeless and classic with a touch of European charm!
Suggestions:
Liesl
Lilibet
Elodie
Caradoc
Desmond
Rhys