My wife and I have shared a Spotify account for ages, and we both like to add songs and listen to various playlists. I'm also sharing the playlists with others but only so they can listen to them and not add songs.
Now I'm sick of not being able to use Spotify when my wife is listening on her way to/from work, so I signed us up for Spotify Family.
But now we have a dilemma: I can share all the same playlists with her, but I can either make a playlist collaborative for everyone or readonly for everyone, but not a mix of the two.
This means that if I want my wife to make changes to playlists, the people who already follow them can also make changes. And if I don't make them collaborative, she can't add songs.
There needs to be a middle ground, or ideally a per-share/account set of permissions for each playlist. This way I can give my wife write access and everyone else only read.
Alternatively, family members would be in a special pool, controlled separately from public access for each playlist.
Or have 2 different shares - a collaborative and a public/readonly one for each playlist so I can give out different urls to different people.
I don't think there's a simple solution to your dilemma. The only thing I could think of is to create two identical playlists. A collaborative one for you and your wife, and another to share to the public.
Still, I like your idea a lot. You should post it in the Idea Submissions forum and it may be considered for future versions of the app.