CMU has a course about building your own dbms and is what you're looking for
https://15721.courses.cs.cmu.edu/spring2019/ note there is YouTube lectures, also check previous years offerings. It focuses on Peloton but numerous asides each lecture about pitfalls, what architecture decisions won't sell, what is bound to be a failed design strategy etc.This was all in the 2017 lectures I watched but I assume the most recent are similar.
