If you think that its finrot and it does sound like it and you have treated it with the more gentle remedies but it has returned I would go to furan2. It works against most bacteria and generally when finrot is on its second round it is likely to be bacterial, and a lot faster working. Its best to move the fish as you can use less sachets to treat. Furan2 has a warning as it has been found to cause cancer in mice etc so you need to be careful when using it.
The belly of your fighter looks okay to me, the yellow sprinkles you are referring to could be small holes developing in the fins. If you look closely you may see a pinky red waterstain looking line up above where the fins and tail are tatty. That is a guaranteed sign of finrot, that is the next point where they will fray too and is definitely bacterial if it shows that.
I would treat with furan2 which is a 4 day course, keep the water as clean as possible as well.
I'm going on holidays today but good luck.