Big congratulations!
My first instinct was to say [name_f]Tilda[/name_f] as it’s my personal favourite from your short list but on second thoughts, [name_f]Allegra[/name_f] feels like a better energy match with [name_m]Cosmo[/name_m]. While both have that chic, European vibe, [name_f]Tilda[/name_f] seems sweet and a little reserved beside [name_m]Cosmo[/name_m], whereas [name_f]Allegra[/name_f] feels lively and vibrant. I wouldn’t normally describe [name_f]Tilda[/name_f] that way, but something about the combination of vowels and consonants in [name_m]Cosmo[/name_m] gives the name a big presence and [name_f]Allegra[/name_f] stands up to that. I would suggest [name_f]Greta[/name_f] as a blend of [name_f]Tilda[/name_f] and [name_f]Allegra[/name_f], but your final options are great.