This is what I've been doing! Pretty much creating a modeling agency first and foremost and a music group second LOL.
However, I believe the idols acting unhappy and leaving the group is a new addition to this update (?) I've never seen idols act unhappy about not having music and concerts. Very interesting! When I did this in past updates the idols were happy to sit there and work as models more or less. I am kind of excited to see them start acting like actual people rather than money-making robots in upcoming updates haha.
I also feel the newer update has made things like releasing singles and concerts more expensive. In the past, I'd be sitting on piles of cash once I started having monthly concerts. Now, it can be really hard to upgrade to the next concert venue size even when I have some cash sitting around. Releasing physical single + concert + election is really expensive too!