Going to give myself this as a challenge
If you found a solution please share it though.
One way possibly is to pass the related item to an include and then run a query in that… I’m thinking using something like the @query directive from this GitHub - Log1x/sage-directives: A set of Blade directives for use with Roots Sage.
(I wouldn’t output the whole slideshow in one in that case, I’d loop over the slides in my template)
However if you want to do it all from your controller slideshow array, you’ll just need to build up an entry like you do for your related post . So instead of
'caseStudy' => $p['slideshow_case_study'], you’ll probably want that to be a nested object/array
An example of the latter is here
https://discourse.roots.io/t/acf-repeater-content-within-a-query-loop-array-map/13270/5
I’ve not tried it but it seems to match what I envisaged.
Will give you an update on the above later in the week