how to limit caption in gallery?

I have a lot to say in my image captions, but showing it all in the gallery looks bad. I’d like to limit the gallery captions to a single line with an ellipsis or maybe a [read more] or something, that when the image is clicked on it opens in a lightbox (already installed) and the full caption displays below the image in the lightbox (as it already does).
So no changes to the caption anywhere else, only when displayed in the Gallery.
How would I go about this, please?
Thank you!

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