Completely customize admin menu without plugin
<p>Basically what I want is to remove most of the admin menu and replace it with my own, without using a plugin. But I'd also like it to stay after a wp update.</p>

<p>Is this a possibility and if so, how?
Or is it just a million times easier to make the plugin and be done with it?</p>

<p>EDIT<br>
In /wp-admin there is a menu on the left side. I won't be needing most of the menu and therefor don't want others to see or edit stuff in there(because the only thing that will happen is it will break the site). So I want to remove the unnecesary menu items and add relevant menu items. (It's about Posts, Pages, Media etc)</p>