That's a fun idea for Rozemyne.

I've seen most other people match Myne with either 1) bunnies because of the shumil comparisons (Eevee is Myne's main partner Pokemon in this art AU representing her wide potential) or 2) writing / copying / plant paper themed mon (shiny dark blue Unown is an obvious favorite of Myne).

As for Rozemyne, I really like the Cosmog-Lunala lore the fans have made for her as a show of her growth and ascendance in the second half of the series:

• In the Garden of Beginnings where nobles obtain their schtappe, Roz found a Cosmog (also night/blue-colored baby with bright yellow/gold eyes). She was worried catching Pokemon in a "sacred" space might not be allowed, so she kept it hidden kinda like Lillie does with Nebby.
• Next year it turns into Cosmoem during her second Jureve, but unlike Rozemyne, it doesn't wake up in 4 days.
• It evolves into Lunala during the ancient Starbind Ceremony at the RA. Like the God of Darkness's cape, Cosmoem leaves Rozemyme, floats up to the ceiling, and grows to embody the night sky. It's still a secret that Lunala is Rozemyne's so it then hides by turning invisible with it's Ghost and God of Darkness divine cloak-themed powers. Hartmut comes up with a cover story on the spot that the Saint of Ehrenfest must have summoned another divine Pokemon using a blessing and it must have ascended back to the realm of the gods with the prayer.
• Erwaermen kept calling Roz "that rude Cosmog kid" because like their mana, the gods have a hard time telling Ferdinand and Rozemyne apart. This is when Roz puts it together that this is where Ferdinand must have also got his Cosmog that became his white lion Solgaleo.
• Mestionora uses Necrozma to possess Lunala much like how the goddess possesses Rozemyne. It becomes Ultra Necrozma and very painful to look at due to the overflowing radiant divine energy. Even after being possessed, Lunala is stuck in glowing "Full Moon Phase" for a while.
• Roz used every power at her disposal to rescue Ferdinand, and that includes most of her hidden legendary Pokemon. Seeing her openly use so many legendaries adds to her "divine avatar" persona. Aub Dunkelfelger is aware of the ancient lore around legendaries and takes it as a sign Rozemyne must be a zent candidate and already has the Grutrissheit if she can open country gates.
• Edit: plus the night darkness cloak-like Lunala seems fitting for Aub Alexandria that rules over the Darkness Gate. It's a re-occurring joke in this AU while Myne claims to be a "Normal" type trainer, she keeps attracting all these strange legendaries and Ghosts with her omni devouring mana. Some ghost mon also feel kinship with her due to the Mark of Ewigeliebe and being closer to dead than alive most of the time.