A lot of CDM is already baked into CDS: Contacts, Accounts, PhoneCalls, etc.
Other parts are omitted because they are foundational to MSFT's D365 Apps (Leads, Opportunities, etc.)
If you want to adopt the model, but don't want MSFT D365 app functionality, then look in the Samples dir of the CDM GitHub. It will not completely solve your problem for you, but it will provide a framework you can easily follow and all you will have to do is insert the correct metadata messages (link).