That's true. I designed those quests fairly early in development without giving enough thought to the overall player experience.
In future updates, I plan to remove quests that require unlocking a specific character, or introduce systems that make them easier to complete. For example, if you have Seraphine's quest active, the game could guarantee that your next pity gives you Seraphine.
I'm still considering the best approach, but I appreciate the feedback. Thanks! ^^