It’s normal for scissors to have different angles on the thumb arm than the finger arm, depending on their intended use. You shouldn’t have to work the backs too much, but they should be revisited every so often during filing/honing to work the burr that is formed by the sharpening. I personally feel that a stone is more economical but you can finish hone with high grit automotive sandpaper too. If you’ve got an old leather belt, some scrap wood, and some glue you could make yourself a strop and only need some polishing compound. The finish work with a strop removes the burr, leaving the sharpened edge - if you don’t remove the burr your tool is likely to lose the edge more quickly.