How to add menu to Dashboard that can be viewed by all users

You have to use the right capability for this. You chose manage_options, which by default only users with an Administrator user role have.

So, change it to read or exist, for instance, and every user will be able to see and access the menu.

add_dashboard_page( 'custom menu title', 'Test', 'read', 'custompage', 'my_custom_menu_page', plugins_url( 'test/images/icon.png' ), 6 );