When you say,
.. that does a simple loop through the ACF Gallery data structure and renders out a thumbnail gallery.
makes me think you’re doing something outside of what a standard WP loop would do, so also outside any type of standard pagination that could be used, so it’s going to need to be custom built.
As an alternate, and this is just a suggestion to think about, Don’t know how far you are into this thing, and it might be too far along to change plans, but, I would use a custom post type with an archive, use pre_get_posts to set up how many to show, and then use standard WP pagination and let WP take care of the details.
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