Thank you for this feedback. I'm happy to announce our new space navigation experience will be released in the coming months, you can learn more about it here - https://community.atlassian.com/t5/Confluence-articles/A-brand-new-space-navigation-experience-is-coming-to-Confluence/ba-p/2423015. While we haven't yet built a tool for customizing the content tree itself, this new experience has a lot of UI changes to modernize the look & feel of the sidenav with tighter density and spacing. It also simplifies a lot of the core experience and adds sections to the sidebar to make it easier for teams to expand and collapse shortcuts, content, and apps. We plan to continue to make the sidebar easier to use so customers can find & access their content more effectively.