You could use a single edged razor blade to cut down the cardboard flashing. This stuff has to be fit first before being glued, If you tried making one of these models, with the intent of painting it, and adding the panel lines manually, it would be a large task, but then you could use filler, and make a perfect hull. The lines would not be as hard as you think they would be. You could use Blue Masking tape, which would not affect the paint, and that would be your edge for making the panel lines. I wide piece of paper, maybe 110 lb. cardstock around the formers would really help in getting rid of the skeleton effect that comes from having such a robust (wide, thick) frame, but not beveling the edges, which causes them to stick out if you use thinner paper when skinning the model. You can still save this and quite nicely, which I'm sure you will do. Each one is coming out vastly improved from the prior one.
That is great progress.