I was so set to suggest [name_f]Tess[/name_f]/[name_f]Tessa[/name_f]!
@kjh446 has some amazing suggestions, such as [name_f]Cleo[/name_f], [name_f]Calla[/name_f], [name_f]Gaia[/name_f], [name_f]Maud[/name_f], [name_f]Livia[/name_f], and [name_f]Elsa[/name_f]. I also love [name_f]Daphne[/name_f] and [name_u]June[/name_u] - strong, feminine, classy, but not frilly.
Other suggestions:
[name_f]Clara[/name_f]/[name_f]Sara[/name_f]/[name_f]Mara[/name_f] - They’re well-known but I don’t think any of them are popular. [name_f]Lara[/name_f] is another option (but I don’t really like it and much prefer the others!)
[name_f]Ivy[/name_f] - getting quite trendy but fits perfectly into your category [name_f]IMO[/name_f].
[name_f]Tamsin[/name_f] - off-beat, unexpected, and rich with history.
[name_f]Judith[/name_f]/[name_f]Judithe[/name_f] - beautiful name that is seriously overlooked.
[name_f]Matilda[/name_f] - for me, the epitome of feminine-but-not-frilly!
[name_f]Bess[/name_f] - [name_f]IMO[/name_f], the loveliest nickname for [name_f]Elizabeth[/name_f]. It can stand on its own though. Very rarely heard these days.
[name_f]Isolde[/name_f] - meaning “ice ruler”, this is one of my favourites. Possible issue with pronunciation - iz-OLD-(uh) is how I’d say it, with the ‘uh’ blurred into the ‘d’.
[name_f]Maura[/name_f] - love Nameberry’s description: “Somber, almost mournful name that still has a lovely sound.”
[name_u]Teagan[/name_u]/[name_u]Reagan[/name_u] - not my cup of tea, but it might be yours. [name_u]Teagan[/name_u] (or [name_u]Tegan[/name_u]) means “fair”, [name_u]Reagan[/name_u] (or [name_u]Regan[/name_u]) “little king”.
[name_f]Meg[/name_f] - I was going to suggest [name_f]Megan[/name_f], but it’s very well-used, particularly in [name_m]Wales[/name_m]. [name_m]How[/name_m] about [name_f]Meg[/name_f]? Sweet, strong, lovely, and distinctive.
[name_f]Margaret[/name_f] - talking of [name_f]Meg[/name_f], there’s also [name_f]Margaret[/name_f], which could serve as a longer form while also being one of the most alluring and regal female names around.
[name_f]Ffion[/name_f] - means “foxglove”. Very popular in [name_m]Wales[/name_m] but I don’t think it’s as widely known outside as I feel it is. (I’ve known several so it’s a familiar choice to me.) Pronounced [name_u]FEE[/name_u]-on.
[name_f]Margo[/name_f] - yes, I know, heavy on [name_f]Margaret[/name_f] variations! This is a spunky choice. I prefer [name_f]Margot[/name_f], but it feels very delicate and flowery to me and that doesn’t feel like what you’re looking for.
[name_f]Gwendolen[/name_f] - with the lovely nickname [name_f]Gwen[/name_f]! One of my more recent crushes 
[name_f]Mercy[/name_f] - I just wish more parents chose this! It’s so perfect.
[name_f]Katia[/name_f] - a Russian diminutive of the (awesome) [name_f]Ekaterina[/name_f], I find this very attractive and exotic but still wearable and actually fairly familiar.
[name_f]Maggie[/name_f] - I don’t know how you feel about nickname names, but [name_f]Maggie[/name_f] is just cool. I personally prefer [name_f]Magdalen[/name_f], especially as a long name.
[name_u]Cedar[/name_u] - Unusual, yes, but it would so work on the right person!
[name_f]Nina[/name_f] - may be more ‘pretty pretty’ than you’re going for, but I think it’s gorgeous.
Annet - so much prettier without the extra -te [name_f]IMO[/name_f]. This is mysterious and charming.
[name_f]Signy[/name_f] - a lovely Scandinavian name pronounced [name_m]SIG[/name_m]-nee (I believe).
[name_f]Bette[/name_f]/[name_f]Betty[/name_f] - full of life and verve.
[name_f]Astrid[/name_f] - means “divinely beautiful”. Another Scandinavian name. And while I’m at it, what about [name_f]Ingrid[/name_f]?
[name_f]Scarlett[/name_f] - Modern, and not at all feminissima. Another strong name. And with the added bonus (or burden) of [name_f]Scarlett[/name_f] O’[name_f]Hara[/name_f].
[name_f]Sian[/name_f] - again, much more popular in [name_m]Wales[/name_m] than anywhere else. Pronounced shahn.
[name_f]Esther[/name_f] - Amazing namesake and amazing name. One of my favourite Biblical choices. (ESS-tuh.)
[name_f]Juno[/name_f] - If you like [name_u]June[/name_u] and [name_f]Cleo[/name_f], there’s also [name_f]Juno[/name_f], if you don’t mind the goddess association.
[name_f]Gemma[/name_f] - [name_f]Georgia[/name_f] is another very similar name.
[name_u]Sage[/name_u] - Technically unisex, but I love it for a girl.
[name_f]Cora[/name_f]/[name_f]Dora[/name_f] - pretty, vintage gems. [name_f]Cora[/name_f] is very stylish at the moment.
[name_f]May[/name_f]/[name_f]Mae[/name_f] - I just love this. Short, but so full of character!
[name_f]Carys[/name_f] - means “love”, pronounced CAH-riss.
[name_f]Kirsten[/name_f] - pronounced KUR-stuhn. Scandinavian variation of [name_f]Christine[/name_f].
[name_f]Hazel[/name_f] - this actually feels quite unisex. Nature name that’s gaining a following, especially among fans of TFIOS (The Fault In Our Stars).
[name_f]Petra[/name_f] - I love how exotic this feels.
[name_u]Blair[/name_u] - this isn’t my style at all, but maybe you’ll like it. It is unisex. [name_f]Claire[/name_f] is much more to my taste but is also quite popular (and has been popular, too.)
[name_f]Imogen[/name_f] - one of my favourites, although it’s certainly very popular on Nameberry!
[name_u]Sloane[/name_u] - NMS but as before, you might like it.
[name_f]Nell[/name_f] - Pretty and sweet. Could be a nickname for [name_f]Eleanor[/name_f]/similar or on its own.
Mhairi - a Scottish form of [name_f]Mary[/name_f].
[name_f]Noor[/name_f] - I’ve had a real crush on this. An Arabic name meaning “light”, it conjures up images of deserts and sheikhs and silken tapestries and caravans and camels, all mystery and beauty!
[name_f]Ilse[/name_f] - pronounced like [name_f]Ilsa[/name_f], a Dutch variant of [name_f]Ilsa[/name_f].
[name_f]Irene[/name_f] - I prefer this spelt [name_f]Eirene[/name_f].
[name_u]Indigo[/name_u]/[name_f]Indie[/name_f] - less common, certainly, and this could be your style. Unisex but it would sound good on a girl.
[name_f]Cleo[/name_f] - I know, I have already acknowledged this, but it seems such a perfect fit! [name_f]Clio[/name_f] is a separate name, more stand-alone than [name_f]Cleo[/name_f] (which is of course from [name_f]Cleopatra[/name_f]) and means “glory”.
Finally, sorry for any repeats - I have had to keep coming back and adding to this so I haven’t seen most of the posts in this thread as I started this this morning! [name_f]Hope[/name_f] it’s been helpful 
xxx