Congratulations on your little one!
[name_f][/name_f]I would go with [name_m]Miles[/name_m] as it has the same friendly, breezy vibe as [name_m]Sonny[/name_m] and [name_f]Ayla[/name_f] and adds some new sounds to the mix. [name_f]My[/name_f] runner-up would be [name_m]Ollie[/name_m] which feels exactly like a brother name to [name_m]Sonny[/name_m] (gives me surfer vibes) but it shares similar sounds with both [name_m]Sonny[/name_m] and [name_f]Ayla[/name_f] which puts it in second place.
[name_f][/name_f][name_m]Ellis[/name_m] and [name_m]Spencer[/name_m] feel preppy to me so not my first choice. I think [name_m]Oakley[/name_m] works but it makes the sibling set feel a very on-trend and loses that breezy vibe for me.