I'm designing an internal application with huge 3 level deep tree structure (attached below) with more than 30+ menus(on first level). The basis operation is select menu item/s.
In case if a user select a parent, all children will be selected by default, however, deselection, customization of selection is permissible.
What I'm thinking to improve the below structure
1: Adding Miller column but since my user are old window user, not sure how much comfortable they would be. Even if it easy , would it be a good idea to add it with real estate restriction. The image below is just a part of a big form.
2: Tag based searching and selection process but since tags are independent with each other, I really don't know if it is a good idea. Other disadvantage is tags will take lots of space.
3: There is no search feature, that can be added to make it more usable. Is there any other suggestion to improve/change feature.
PS: I've blurred the actual content due to privacy, However, I'm sure you know what i'm looking for.