I’d say the ideal thing to aim for is case 2-ish. Building in isolation isn’t great and dog-fooding your lib (via their projects) can help reduce the amount of future breaking changes (getting things right the first time). I.e. Ideally your helping people upgrade when needed and seeing their problems. Even if your forking their projects to try your experimental changes out for them and then provide a PR? That’s probably a lot easier said than done though xD

A dependecy management system to let consumers know when a new version is available could go a long way, or you pushing for them to update by talking to them (as this is all internal?).

Basically reducing the distance between teams and getting the tightest possible feedback loops should be the goal.

Thate my rushed 2p while waiting for a haircut anyway ;)