Yep, indexing is to make it easier to make sure your fingers are in the right spot. Most keycap sets will include indexed versions of F and J for QWERTY layouts, although you can usually also buy additional indexed keys (such as T and N) for other layouts. The traditional indexed keys help with horizontal alignment while the traditional row stagger helps with vertical alignment. However with columnar stagger like most split ergo keyboards use I actually find vertical alignment to be the bigger issue most times since all the rows “feel” the same. Having some bumps on the top row would help with the vertical alignment I think.

Looking at the options though I’m not terribly thrilled with the colors/design/looks of most of the ones I’m finding. I think you found the perfect ones for your caps and it just all looks so clean (the alignment of those stickers on the keys is just chefs kiss perfect, well done). Unfortunately if I picked up a voyager I was thinking about replacing the caps with the MBK Legend keycaps and I’m having trouble finding anything that would look anywhere near as nice as your setup with those.