Amila Laravel CMS is a good choice for building/integrating a CMS into an existing Laravel 5/6 project. The features for an existing Laravel website or project are:

  1. Can define a database table prefix like laravelcms_   cms_, won't mix with your existing tables
  2. Do not have user/role/permission tables, using your existing Laravel user & role system
  3. Install as a vendor package, won't mess up your PHP scripts
  4. Include an uninstall feature, it will automatically delete the installed database tables and related files(views, upload files, assets) 
  5. Built-in bootstrap 4 templates with the same Laravel blade template syntax 
  6. Very flexible for the developer to build your own templates or plugins
  7. Built-in JSON API can be enabled in 10 seconds
  8. Free, open-source under MIT license, can be integrated into your commercial project.