Haven't cut into stringers or a transom but I have worked with
fiberglass. One of the ads I posted looking for a project boat
requested no hull damage. The only way I would tackle that is if the
boat was worth a lot more than most of the abandoned boats I've seen.

That would be some tough work for sure. Using a grinder on anything
takes a long time. I was reading about using a special chisel that
would cut through it really easy. That would be the way to go. But
yeah, I'm trying to stick to any kind of repair but that.