Hide SubMenu items

Hello dear WordPress users,
I’m relatively new to WordPress. I used to work with Joomla before WordPress, but now I wanted to try something new and migrate my old site to WordPress.

Everything worked fine.
But now I’ve encountered a previously unsolvable problem.
I would like to hide submenus. These are relatively easily set to “future” in the “post_status” column in the database under “wp_post”. As a result, they no longer appear on my site. That’s the way it should be. Later, when it is needed, I will set it to “publish” via PHP script and everything is perfect.

But now my big problem. When I create my menu or change something in my theme’s menu editor and then click “save menu”, all menu items are set to “publish”, although I may have only changed a small part of the existing menu. This is very bad, as many submenus are still hidden and then suddenly appear.

I desperately need your help.
CSS with “display:none!important;” unfortunately didn’t help either, although I wrote a corresponding class in the “style.css” and the “li” tag also contained the class reference.

I hope there is a good solution to my problem.

Best regards,
Your Nico