Low-Key ToolBar makes the block editor toolbar less noticeable and improve block editor UX.
I think block editor toolbar is useful, but too big and too conspicuous and disturbing. Sure, top toolbar mode is not disturbing, but is not user-friendly.
This plugin add its own setting sidebar to block editor and You can control scale, opacity, vertical position of the block editor toolbar.
By adjusting these properteis, the toolbar will be less noticeable, and it will improve UX.