Maybe use the script from the getkirby.com repo instead, it has a built-in prev-next option that you can use by passing a group name to the thumbs, e.g. data-lightbox="gallery".
You can find the unminified original code of the lightbox scripts in the getkirby.com repo if you want to build on it:
Edit: I thought both scripts were identical, but they are not.
This seems to be the script used in the Starterkit (doesn’t look as if there was a lot to modify).
If you need more control or features, maybe a different lightbox library would be a better option.