Customize the content manager

I a building a system for digital courses.
Each course is composed of lessons, and each lesson is composed of tasks.
I have a team of content creators, and I want to expose the content manager to them for them to add their content.
My problem is that the relationship field in the content manager is hard to use, and I cannot create a new item in the relationship inline in the forum.
For example, I want to create a course that has 2 lessons, then when I create the course, and try to link the lessons, I will see a list of all the possible lessons, and the system has too many of them, and there is no option to filer.
How can I create a relationship field that has a custom ui, or even better a relationship field that lets you create a new item in the associated collection, and edit it.