I thought graphics design was what was needed, but it doesn’t seem the best for learning retouching.
Graphic design is more about the formal design aspects: Color use, composition, typography, layout. It only teaches how to integrate photos into bigger projects, but not much about working with photos themselves.
A retoucher would get a lot of benefit from taking fine art drawing and painting classes. These would teach you the basic skills of retouching independent of any software: How do you touch up an image so that your edits look real and believable? What do you need to know about light and form? Training in software or design won't help you there. You need to know how real objects respond to light and appear in perspective, and how to render that with any material (paint, pencil, digital...). You should know the basics of figure drawing and anatomy for retouching a face or body, how fabric drapes over a body. Maybe a little about landscape painting.