Today I finally found out how to easily customize the Admin Menu and the Admin Toolbar for the Editor role of my clients, so they only see what they really really need. There are plugins doing the same and even much more, as the Adminimizer, but it offers too much for my simple needs. For most clients I need just a few lines of code for my functions.php. Reading through the WordPress Codex was enough, and here is what I did:

Just add this code into the functions.php of your child-theme and adapt it to your needs. I left all the default items and just commented them out with //. In this way we can easily adapt everything specific to each client by commenting/uncommenting. The last line is only relevant for the DIVI Theme.

Find more information on customizing the Admin Menu in the WordPress Codex on these pages:
https://codex.wordpress.org/Function_Reference/remove_menu_page
https://codex.wordpress.org/Function_Reference/remove_submenu_page

Find more information on customizing the Admin Toolbar in the WordPress Codex on these pages:
https://codex.wordpress.org/Function_Reference/remove_node

Find more information on conditioning functions according to the user role or capabilities:
https://codex.wordpress.org/Function_Reference/current_user_can
https://codex.wordpress.org/Roles_and_Capabilities

Enjoy creating your perfect WordPress for your wonderful clients. If you want them to have access to the Widget and Menu area without giving access to the whole Theme Customization, this article might be interesting for you.