Your problem is not the wood but the fiberglass. Here's why.

You have a white band because the glass strands were shattered. You have to sand that smooth but if you sand through it into the wood, the new glass wet out will probably not match the color of the surrounding wood. This occurs because you will probably never be able to match the sanded texture you had when you built it.

If most of the tear did not penetrate the wood, you can try to sand the white scar and before you go through the glass, fine sand the scar with maybe 200 grit wet and see if you can get it to wet out transparent. That's the ticket for a transparent repair. If it really did rip into the wood, then you'll have to sand the glass fine and I would just pour in some epoxy in the WOOD scar and one that sets, glass over that. You will probably see a dark line but I think it will be the less of the evils.

I have done repairs similar but not as long as yours. There's no one positive, perfect way to do it other than some experimenting as you work.