THE NEXT AYRA

Try it the other way round and use flashing blade as a seal.
I personally take for the A-Slot LnD for the glass canon build or Brazen ATK/SPD.

Why is flashing blade needed for a 1-charge special?

I don't know about that, Glimmer is worse than Moonbow with Navarre's atk stat, it will be usefull only against greens.
I don't think he can reach Ayra level of stregth, he strength comes from the fact that her bulk is enougth to let her activate Wrath empowered Regnal Astra twice in a fight dealing around 26 dmg each time just from special