Fiberglass repair on keel

[Mattm]

I would not start small and end big unless you were planning to vacuum bag the repair. And even so It will add too many voids between layers that are purely resin. Which will result in a substandard repair.
If done right the repair will not fail. If you are looking to add insurance that can be done from the otherside of the glass.
As to the ops question. That damage is actually quite a large repair. It is not structural so I would suggest building up with mat this will take about 4-6 layups to prevent overheating. You could potentially build up with loose glass fibres but due to the size I would not recommend using any of the powdered fillers at all