Oh I also had Fia/Fiadh on my heart for my first daughter but ultimately didn’t use it for the same reasons you listed. We went with Flora instead. However with your boys and naming style I think it could really work. You already have a P@x for example, 3 letters and zippy. Something like…
Fia Leonora Maeve
Fia Jolie Marigold
Or in the middle somewhere
Matilda Everly Fia
Everly Fia Odette
I love Aurelia for you too on the first or middle. Suitably grand and bright! Ooh or what about Ottilie? Or Cordelia for another Shakespearean name with a lovely meaning. Perhaps Odessa or Isadora for that darker flair.