I wish I could look over your shoulder and see how you have organized your notebooks, But you don't need tags at all for what you want to accomplish. I have a notebook dedicated to my business. One of the sections is titled "Clients." Each page in that section is devoted to a single client. Each client page is followed by subpages with subordinate subpages. I am careful about naming pages because OneNote has a powerful search engine. Pages, subpages and text boxes within pages all carry important keywords. Because of this scheme, careful naming, and keywords in text boxes within pages, I can search out any information I need without flags. I do use flags, but they are task oriented.
The key to organizing information within OneNote is discretion. You don't want to mix up different kinds of information on any given page. It is better to place information about a master page (the client's page for instance - but any master page) on subpages and subordinate pages. If there at 10 distinctive things about a particular client, there should be 10 subpages dealing with those things.
For maximum searchability, you want your section topics very broad and general then zillions of pages grouped by master pages with subpages and subordinate subpages so that you can limit your searches to "This Page" and "This Section." In your Business notebook you will have just one section on marketing, then lots of pages grouped by the different kinds of marketing; a section titled "Team" with all the pages you need to keep up with your stakeholders from employees, to city officials, to investors. The topic "teamwork," however, would be in your reference notebook. The business notebook would be devoted just to the details of getting things done. The "About" topics and guidance for how to do those things belong in reference.
There is an important hierarchy to keep in mind. First, very few notebooks. My operation is very, very complicated and I have only 3 notebooks (Business, Personal, and Reference). Secondly, very few sections within notebooks (only 5 or 6 sections per notebooks). You need to see sections and page titles at a glance. So third, pull your page title column toward the center of the page so you can see at a glance a good portion of the titles. My page section takes up almost half of the right side of the window. Use only one word to title sections and you can fit 5 or 6 sections in this layout.
For business, for research and writing --- any project that is information intensive and where that information must be searchable, you need to keep your text boxes narrow and your page titles view wide, wide enough to read at least 10 or 11 of the words of your titles. This means, if you want to use OneNote for planning --- if you need a wide work page where you can put photos and drawings and tables and mind maps and so forth, you need a separate planning notebook where the page titles column is very narrow.
Finally, if you decide that you need to reorganize a notebook, don't do it in that notebook. You will get lost in the shuffle. OneNote has an amazing feature. Right click on any page and you will see "Move or Copy." Right click on any section and you will see "Merge into Another Section," in addition to "Move or Copy." Begin reorganizing by copying each section into a new notebook. When your are finished, it should look exactly like the old notebook, but don't delete the old one just yet. In the new notebook, begin first by retitling your sections and merging them down to just 5 or 6, and then move all your existing pages into the appropriate sections. Begin deleting and shuffling your pages around, creating new subpages until everything is organized for search. If you get confused, use the old notebook for reference and remembering. If you screwed something up, you can always get it back from the old notebook.