Sorry to hear about the pleco.
Having had silver dollars in the past, they are pretty hardy, but when they do get sick it usually is something external like that. Almost everyone of the red hooks I got seemed to get stressed and get fin rot. Some responded well to Melafix and Pimafix treatment but it’s something I dont really like to do because it isnt real ‘medicine’ so to speak and it reduces oxygen in the water. Ive seen fish struggle to breath using this treatment. Could add air stones to help counteract.
When I have actually used Medication, I found Seachem Kanaplex to be very effective since it can be absorbed through the skin. Or it can be mixed into food with Seachem Focus which is a binding agent. The feeding method can be super effective because of the aggressive feeding nature of silver dollars. If they are still eating, then the kanaplex food mix gets inside them and seems to work even more effectively.
The blue dye treatment is also something Ive done but usually only as a bath. I know you pulled your biological out to treat since the dye will kill that. Im surprised that the dye treatment did not help... there is another treatment that involves the use of hydrogen peroxide used in a bath. That would be more last resort. I would say the Kanaplex route would be the method I would take first.