I continued using the scotchbrite wheel, files, and speed deburrer as previosuly but instead ot the dremel tool, I wrapped a piece of sandpaper around a wooden dowel. I would then swing this around on the inside edge of the lightening holes in the ribs and it seemed to work much better than the methods I'd tried until now. Still a fair amount of elbow grease required but definitely faster and neater. The sandpaper doesn't last too long and it does appear to work better if there are at least three to four layers around the dowel.
Still a time consuming process but there doesn't seem to be any way around it - I'll just slog through it and make that all edges are smoothed and rounded as I know I'll be forcing my arms through those holes in the future and have no desire to "bleed over the project"!
Sheet of sandpaper wrapped around a wooden dowel
All ribs without the little sticker on are complete
