I am trying to separate the following part along the red and green surfaces.

The problem is that this line is not located on a single plane.

If I use convert entities on the front plane (or any other plane for that matter), the cut will completely mess up the inside edges and not cut completely through as the cut planes are not aligned.

I want the cut line to be like the black line on the edge shown above.

Is there a way to make the cut follow multiple planes?

I thought about using a custom reference plane for every single segment of the cut but that seems very ineffective.