I love the name Léa. Here are a few suggestions based on the names you like already and your criteria:
[name_f]Arabella[/name_f] - Simply on the basis that it’s long and elegant, it has the “el” sound you seem to like, and it feels strong (at least to me – I read somewhere that some interpret [name_f]Arabella[/name_f]'s meaning to be “beautiful lion.”)
[name_f]Eleanor[/name_f] - Feels very similar to [name_f]Elizabeth[/name_f] in terms of style and because of the “[name_f]El[/name_f]” at the beginning.
[name_f]Elodie[/name_f] - [name_f]Elodie[/name_f] feels feminine and elegant like [name_f]Elizabeth[/name_f] does, and it shares the “[name_f]El[/name_f]” beginning, but it’s far less popular, both in Europe and in the United States (though in the UK it is in the top 200).
[name_u]Eloise[/name_u] - Another name that starts with [name_f]El[/name_f] and feels elegant. (It’s funny that elegant starts with el, too!)
[name_f]Estelle[/name_f] - [name_f]Estelle[/name_f] is a relatively uncommon name, and it has the “[name_f]El[/name_f]” sound from [name_f]Elowen[/name_f] and [name_f]Elizabeth[/name_f] that you seem to be drawn to.
[name_f]Francesca[/name_f] - I thought of this based on the fact that you said you liked names that were feminine but still strong. [name_f]Francesca[/name_f] fits that description well. [name_f]Francesca[/name_f] also has a variety of unisex nickname options, like [name_u]Frankie[/name_u] and [name_f]Frances[/name_f]. It is relatively popular in Europe, but it’s only in the top 20 (or even in the top 80) in [name_f]Italy[/name_f].
[name_f]Genevieve[/name_f] - [name_f]Genevieve[/name_f] feels elegant yet strong, and it pairs nicely with the names on your list: [name_f]Genevieve[/name_f] Léa, Léa [name_f]Genevieve[/name_f], [name_f]Isla[/name_f] [name_f]Genevieve[/name_f], etc.
[name_f]Juliette[/name_f] - Again, thought of this because it feels elegant and strong at the same time. Also, [name_f]Juliette[/name_f] Léa is a beautiful combination, in my opinion, or the other way around – Léa [name_f]Juliette[/name_f]. You could also consider [name_f]Julia[/name_f], if you like [name_f]Lea[/name_f]. [name_f]Julia[/name_f] [name_f]Elizabeth[/name_f] is a nice combination.
[name_f]Laurel[/name_f] - [name_f]Laurel[/name_f] has the softness that [name_f]Elowen[/name_f] and [name_f]Isla[/name_f] have. [name_f]Elizabeth[/name_f] [name_f]Laurel[/name_f] is a nice combination.
[name_f]Leila[/name_f] - Feels similar to Léa, but sidesteps the issue of mispronunciation, and has the same -la ending that [name_f]Isla[/name_f] has. You could also consider [name_f]Lila[/name_f].
A few other names I thought of that you might like, though it’s a bit more of a long shot: [name_f]Adeline[/name_f], [name_f]Esme[/name_f], [name_f]Lila[/name_f], [name_f]Ella[/name_f], [name_f]Stella[/name_f], [name_f]Violet[/name_f], [name_f]Georgia[/name_f], [name_u]Blair[/name_u]