[name_f]My[/name_f] big question would be about how you pronounce Shay’s name (like “shy” or “shay-rhymes-with-pray”)? I worked at a daycare and we had one of each pronunciation at the same time! (I am guessing from the inclusion of [name_f]Shiloh[/name_f] that you say it as “Shay-like-pray”, but I will include my notes for both variations).
[name_f][/name_f][name_f]Shay[/name_f] (“shay-as-pray”) and [name_m]Charlie[/name_m], in my mind, might get someone seeing the two to read it as “Char-LAY” (I was reading it that way when trying to think of a sibling set in my mind).
[name_f][/name_f][name_f]Shay[/name_f] (as “shy”) and [name_f]Cassie[/name_f] have a similar sound thing with me, where I might then, if reading them together, think “Shay and Cass-i”.
[name_f][/name_f]Those are both me being picky though! They are pretty names!
[name_f][/name_f]Of your options, I think [name_f]Devon[/name_f] fits most what you said you want. However, a lot of your options listed are longer, well-known girl names with nicknames that are either seen as girly (Cassie and [name_f]Liv[/name_f] are very “girl” to me), and I think at this point although [name_m]Charlie[/name_m] is a gender-neutral name, I’ve seen it used for more girls, and I believe that will continue to be the case with how many Charlottes there are (I don’t recall numbers, but I feel justified in my reasons for guessing that there are far more Charlottes than [name_m]Charles[/name_m] lately).
[name_f][/name_f][name_f]Cassandra[/name_f] [name_f][/name_f]- feels the “oldest” in my mind on your list. Knew a wonderful [name_f]Cassie[/name_f], but I’m not sold on the name. This may be due to my using it as a name of a mom in a story I am writing currently, so I wouldn’t put too much weight into my feelings on that one. 
[name_f]Devon[/name_f] [name_f][/name_f]- I lean more masculine than feminine on this one. Feels very fresh, and sort of like navy blue.
[name_f][/name_f][name_f]Charlotte[/name_f] [name_f][/name_f]- feels very feminine, especially compared with [name_f]Shay[/name_f]. It feels more frilly than the tailored image of [name_f]Shay[/name_f]. With [name_f]Shay[/name_f], they could be masculine or feminine or any mix (at least in my mind). With [name_f]Charlotte[/name_f], even with the nickname [name_m]Charlie[/name_m], at most I think of a sort of “farm girl/tomboy”). There is not the same level of ambiguity, if that is something that you want to maintain.
[name_f][/name_f][name_f]Olivia[/name_f] [name_f][/name_f]- feels more popular than [name_f]Shay[/name_f]. I do like the nickname [name_f]Liv[/name_f]. I think [name_f]Shay[/name_f] and [name_f]Liv[/name_f] would go together better than a [name_f]Shay[/name_f] and [name_f]Olivia[/name_f], but I also wouldn’t use the nickname without the full name. If I saw [name_f]Shay[/name_f] and [name_f]Olivia[/name_f] on a piece of paper (similarly with [name_f]Cassandra[/name_f] and Charlotte) I would think “Oh, they love multi=syllable girl names with strong feminine backgrounds” and guess that [name_f]Shay[/name_f] was a boy name. With a name like [name_f]Devon[/name_f] though, I would probably stop and as the parents what gender the kids are if registering for an event or something.
[name_f][/name_f][name_f]Shiloh[/name_f] [name_f][/name_f]- too similar to [name_f]Shay[/name_f]. If you are using a “shy” pronunciation, then they are WAY too similar, and I
[name_f][/name_f]I think [name_m]Auden[/name_m] might be a good, similar to [name_f]Devon[/name_f] but a bit different (and I believe a bit rarer to hear). I like it s a gender-neutral option, although it could lead to confusion as “Aiden”.
[name_f][/name_f]I have two names that I think might fit even better than [name_m]Auden[/name_m] though (at least for your criteria). [name_f]Beverly[/name_f] used to be a more common boy name, and it has a history of being gender-neutral. [name_f]Bev[/name_f] is a fast, one-syllable nickname that would match [name_f]Shay[/name_f], but the longer full name gives more distinction between the two to keep them from feeling super matchy. The names would also both end with “y” in that case. I also want to suggest [name_f]Lou[/name_f]. It is such a sweet, gentle name (I would especially think it would match Shay/“shy” sweetly. [name_f]Lou[/name_f] is also gender-neutral, although I think it may be as a nickname in the case of many girls.
[name_f][/name_f]McKay, [name_m]Brooks[/name_m], [name_m]Mio[/name_m], [name_f]Rue[/name_f], [name_f]Monroe[/name_f] [name_f][/name_f]- these are more gender-neutral ones that I just like, so figured I would share.
[name_f][/name_f]Oh, and [name_m]Jude[/name_m]! I think this one might be a bit more masculine use than a girl, but I see [name_f]Judith[/name_f] being a strong, long-form option for a girl too.
[name_f][/name_f]I would LOVE to have two little girls someday, so I think you are incredibly lucky. Enjoy! And whatever name you choose will be wonderful.