First time repairing core rot

[Clairebear]

First time repairing core rot, I just cut out the places it felt soft to the touch but it appears the wet wood runs farther, past the bulkhead/maybe to where the window sits. I’ve gone as far as I can easily go, what I’m wondering is to I just stop here and repair the scratches where water was getting in so no more damage can be done or do I need to cut deeper into my fiberglass to fine the end of the wet wood? I’m scared I’ll need to take out my window or cut into the bulk head. Also, it’s marine plywood inside it appears, should I replace it with wood again or would coosa board be stronger? Not sure what the best plan of action is at this point.