Magento 2 – How to Remove an Existing Menu Item

I think you're missing the acl here.

     <menu>
        <add id="Company_Module::dashboard" title="Dashboard1" module="Company_Module" sortOrder="12" resource="Company_Module::dashboard"/>
        <add id="Company_Module::subdashboard" title="Subdashboard" module="Company_Module" sortOrder="12" action="company/dashboard" resource="Company_Module::dashboard" parent="Company_Module::dashboard"/>
    </menu>

and acl like:

    <acl>
        <resources>
            <resource id="Magento_Backend::admin">
                <resource id="Company_Module::dashboard" title="Dashboard" sortOrder="12" />
            </resource>
        </resources>
    </acl>

Basically, your problem lies in the permissions. If you inherit it from Backend, you are using the Backend's permissions, and you are fine. But if you want to have submenu, you need your own acl.

It seems like the menu.xml handles the following actions:

• add
• remove
• update

Thus you can something like this to update an existing menu item:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Backend:etc/menu.xsd">
    <menu>
        <update id="Vendor_Module::test" title="Updated Test" module="Vendor_Module" sortOrder="100" parent="Magento_Backend::content" resource="Vendor_Module::test" />
    </menu>
</config>