Sad news. Nothing much you can do with this short of shelling $1.5K on a proper fix at the body shop and mind you, not many even will agree to do the work as probability of re-appearance of rust is high and they don't want to deal with unhappy customers.
IMO this is a bad design. There is this spot welded and seam sealed seam that connects quarter panel and wheelwell sheet metal. So eventually seam seal deteriorates being exposed to road debris and salt flying off the wheel and is starts to rot from the inside. No amount of effort trying to remove the rust spot from the outside and painting over it will produce any lasting results. It will be just frustration.
Mine has fender flares installed. So what? it has rusted right there in the small area that fender flares did not cover (or I do not want to think about what is the condition of metal under these flares).
So I would leave it alone. You can go to a body shop and have a chat about that a PITA these Honda/Subaru trademark rust spots are (Subaru was even worse). I used to have an '04 Subaru and it had rusted out rear quarters.
The guy I spoke with said that to fix this "properly", they would need the whole rear quarter panel replaced. Which makes it ridiculously expensive. The $1.5K -2K fix I mentioned in the beginning involves cutting out the rusted area and welding in piece of good fender, plus some creative welding from the inside of the wheel well to seal it up. Probability that it will fail in 3-5 years is high.
So the lesson really, these spots need to be taken care from day one the car was new. Not that it helps you much now, but hey, I bet almost all Odys of that generation in the rust belt have rust spots in this area.