Thoughts on your existing list:
[name_f]Nora[/name_f] - [name_f]Nora[/name_f] is such a pretty name, and I think it fits the bill of “strong and feminine,” but in comparison to [name_f]Willa[/name_f], [name_f]Marigold[/name_f] and [name_f]Beatrix[/name_f], it’s very common. If that’s not a priority for you and it’s just a coincidence that the other names are relatively rare, then disregard this 
I love [name_f]Nora[/name_f] as a nickname for [name_f]Eleanor[/name_f].
[name_f]Willa[/name_f] - I like this name, and it seems like it works well with your style: it has the softness and simplicity of [name_f]Nora[/name_f] but it’s more of an unexpected choice like [name_f]Marigold[/name_f] and [name_f]Beatrix[/name_f]. I also don’t think you can get stronger than the meaning “resolute protection.”
[name_f]Marigold[/name_f] - I would love to see this name more often–it’s pleasant and sweet and fresh while still feeling classic. I think it works especially well as a middle name. Would you consider doing two middles? I love the sound of [name_f]Beatrix[/name_f] [name_f]Marigold[/name_f] [name_u]Jean[/name_u].
[name_f]Beatrix[/name_f] - [name_f]Beatrix[/name_f] is great. It absolutely has the strong and feminine quality you’re looking for, especially with the nickname [name_f]Bea[/name_f]. It’s so sweet and spunky and strong all at the same time. And the meaning (“she who brings happiness”) is as good as you can get.
I’d go with [name_f]Beatrix[/name_f], if I were you. I also like the suggestions [name_u]Ruby[/name_u], [name_f]Margot[/name_f] and [name_u]Vida[/name_u]. But if you’re looking for any other options, here are a few I thought of:
[name_f]Alice[/name_f] - I’ve always thought of [name_f]Alice[/name_f] as a companion to [name_f]Nora[/name_f]. I like [name_f]Alice[/name_f]'s strong and substantial beginning and delicate ending; it provides for a nice balance.
[name_f]Celia[/name_f] - I’m mostly suggesting [name_f]Celia[/name_f] because I think [name_f]Celia[/name_f] [name_u]Jean[/name_u] is a lovely combo, but it also feels like a nice compromise between the softness and simple elegance of names like [name_f]Willa[/name_f] and [name_f]Nora[/name_f] and the vibrancy and spunk of names like [name_f]Beatrix[/name_f] and [name_f]Marigold[/name_f].
[name_f]Dahlia[/name_f] - this is a name I always assume people will like if they like [name_f]Marigold[/name_f], because both are flower names with a bit more gravitas than the usual [name_f]Rose[/name_f]/[name_f]Daisy[/name_f]/[name_f]Violet[/name_f] (although those are all beautiful, too!) and because they both feel a little bit vintage. Also, I like the combo [name_f]Dahlia[/name_f] [name_u]Jean[/name_u] a lot. I think the hard D initial lends this name some strength.
[name_f]Ingrid[/name_f] - I find it immensely cool that this name derives from the name of a Norse god (of peace!) and the sound feels solid and strong to me, but it’s made more feminine by the [name_f]Ingrid[/name_f] Bergman association.
[name_f]Freya[/name_f] - Another Norse mythology name…has the same gentleness as [name_f]Willa[/name_f] and [name_f]Nora[/name_f], but the two consonants in the beginning make it feel more strong and substantial to me.
[name_f]Valentina[/name_f] - If you’re looking for a feminine and strong name, [name_f]Valentina[/name_f] seems perfect, because the name itself is so overtly feminine, has so many strong consonant sounds, and means “strength.”