Because the BMC was very well developed in the anime, probably the most characterized villain group rather than just focus on Demande and Black Lady.
The Inners also got a lot of good plot instead of get captured.
The final fight was a bit weak but so was the manga, the climax was Demande's...
I mean among all the big bads, she is the one with the most personality and the most interesting one.
She seems to be a villain with sympathetic past when she showed up, but then the reveal of her past was very shocking because she was just..insane. Simply because of a mirror dream of growing...
Classic: Not much to change
R: Switch battle to the future after Reubus arc. Give Demande more screen time to explain his obsession. Let the sisters come back, along with the redeemed Demande brothers' spirit to help in the final fight
S: Let the Inners and Outers work together in the end to...
Berthier does fit Beth in some way but her dark playful personality was quite unique.
Calaveras doesn't resemble Meg much.
Petz doesn't fit Jo as well.
I do think Berthier and Koan probably inspired Elsa and Anna.
I mean her power is water and as a good swimmer, she fits the mermaid dress perfectly.
But her looks and behavior fits better on the human princess, who is supposed to be more mature and very beautiful.