Totally agree that [name_u]Sunny[/name_u] works as a nickname for [name_f]Persephone[/name_f] - the sounds are all there and in the right order too! [name_u]Sunny[/name_u] is bright and sweet but it’s not a full name to me, and this is coming from someone who is big into informal and nickname names.
[name_f]Persephone[/name_f] is dramatic and elaborate, but there is such a wealth of nickname options (including your husband’s favourite [name_u]Sunny[/name_u]) that I think anyone could wear the name. It’s a gorgeous name and - apart from the story behind it - feels no more “fussy” or “out there” to me than the very similar [name_f]Penelope[/name_f].
I think [name_u]Sunny[/name_u] can be used as a nickname for any little girl. ([name_f]Birdie[/name_f] is another nickname that works the same way [name_f]IMO[/name_f].)
I also came to suggested [name_u]Sunny[/name_u] as a nickname for [name_f]Persephone[/name_f]. I like [name_u]Sunny[/name_u] a lot but only as a nickname. I personally like [name_f]Sonya[/name_f] shortened to [name_u]Sunny[/name_u]. But [name_f]Persephone[/name_f] is gorgeous and it’s your favorite, it has all the sounds of [name_u]Sunny[/name_u], his favorite. It gives your daughter an uncommon but classic formal name and an uncommon but very casual nickname. And she can use whichever fits her in the moment and at that stage of life.