Quote & invoice management only scale so far with basic Excel and Word documents, and copy/pasting product references and prices accros documents will sooner or later fail in some way, leading to headaches and financial consequences. Developing a small web application was to solve those issues, but it was important to keep things simple as replacing a process issue with a complicated, hard to maintain codebase didn't sound like a solution.
This was with this premise in mind that I decided to not build a custom admin interface but instead rely on the battle-tested Django Admin. It certainly has some limitations but provides components which withstood the test of time while taking full advantage of the built-in permission model. In addition, those limitations can also serve as a great reminder that if it's not possible without overriding half of the default implementation, maybe you need to go back to the drawing board and think the problem through.