My idea to solve it is to trim a little surface 'A' using a router, but I am not sure from which direction to do it and with which type of router bit. Is this a good approach?

This video demonstrates the complete process of making door casings from pine wood using a manual router, showcasing various techniques ... Perfect for DIY enthusiasts and woodworkers looking to ...

A trim router is a great finishing tool for when you're done with the broad strokes and need to get into the details. Here's what you need to know.