Family plan allows you to have multiple accounts, but if a single account is logged in multiple devices (e.g. some home device that's playing stuff at home, and your phone in your car) then they would do what OP says... unless they constantly re-log in the home music device (whatever setup you're using) to switch accounts based on who is home.

Oh, right. We do that anyway as we have very different tastes and sharing the same account messes with the suggestions too much.