Looking the drupal, not joomla, wordpress, they are different in design of project structure and refactoring. The drupal is the better than others, maybe you learning and get a good idea from this web system , you know, actually, it is a CMF, not just CMS, what dose it mean CMF, that is CMS and FW. You can read the official documentation that tells how to develop a drupal and how to refactor a function in some situation frequently, in other word, it teaches you how to design, why you need to choose the drupal in the guide documentation it says.
But, all we know, the drupal is slow because it is very fixable and strongly in every function of application. You maybe just need some function as the portion of drupal, not all of them,
You can follow its workflow of system and design your CMS what you need the function in.
Such as you said, how to categorising by tag of post, designing a database scheme, invoking a template of view, searching contents in a CMS, etc
Perhaps you learn its principle, then design a weighlight excellent CMS of your.
Good luck to you.