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If it was reduced in size, it would seem to indicate that it can be buffed out. You will need to use a stronger/more aggressive compound and pad. There are a number of marine specific compounds, but you can try an aggressive auto compound to start. I would suggest: Griot's Garage Boss Fast Correcting Cream and a microfiber pad. Your Harbor Freight buffer is also a weak link. You can use it, but it will take a very long time with many applications depending on the scuff. You would be better served by either: a long throw (15-21mm) RO buffer or a RO buffer with forced pad rotation. Do not skimp on the buffing compound (you want a sub-micron abrasive that breaks down versus a cheap gritty compound that leave lots of scratches) or the buffer.
You also might want to look at Shurhold. They sell a buffer and a compound that many say work well together; they have videos on their site. It might be your least expensive choice.
I agree with @Elliott more buffing. I would try the 3m again later when you clean again. I would not recommend a gritty compound especially on a black boat. I just would not reco that to someone that I dont know thier experience with wheel (high speed buffer) you might haze or burn the gel coat.
Worst case right now, touch up with a sharpie until time to clean again. Although the sharpie might stay with the rejex.
If it is that small, consider just letting it be. It is a big boat. I get that it is your baby (I have one too), but it will get dings and scratches. Yes, we hate it. But, the big ones we should address and the little ones (like this), we should consider them as love wrinkles and move on to enjoying the boat on the water.
There is a practical side to this, too. As you use stronger compounds to rub out this little scratch, you are removing gelcoat. How much of that is worth it? You risk removing enough that the next time (or even this time) you hit the fiberglass. Then you are relaying the gelcoat. Not tragic, but a lot of work for one little scratch.
IMHO, etc. But consider calling the smaller scratch a win and floating on your boat.