my very first thought was [name_f]Margo[/name_f]! easy to hear and read, works professionally and on a child, two syllables, not that popular, and gender identifiable! the only concern would be the ambiguous spelling, it seems! I also think it goes great with big sisters’ names (which are lovely by the way!)
some other ideas:
[name_f]Maya[/name_f]
[name_f]Audrey[/name_f] (although it is in the top 100, it’s 82nd in the US for 2024)
[name_f]Amy[/name_f]
[name_f]Camille[/name_f] 
[name_f]Hannah[/name_f]
[name_f]Anya[/name_f]
[name_f]Brynn[/name_f]