Mimi: Genuinely no clue because it's part of his childhood too... probably some sort of other entertainment industry job like an actor
Silver: Creative writing professor
The other three do end up pursuing other careers! They just do the band gig as teens/young adults while somehow juggling with irl life and school at the same time lol anime logic but here are their careers post-timeskip
Yura: Pharmaceutical research
Delta: Concept/character design artist
Lucius: Undecided between pediatrician and surgeon for now but some sort of doctor for sure