I ended up changing a part of the storyline in the background, because my first idea would go too smooth, easy, and boring, since it would be avoiding conflict.
The finale of this campaign remained unchanged in my head for years. I still like it.
Adding Trixie and Starlight will lessen confusion in dialogues, because Twilight would end up playing at least 4 speaking characters in one scene.
Also I got a bit bored of the “only those 4 specific ponies” limit in this D&D storyline.