When I started using OBS in 2021, the Slideshow source loaded all your images into video RAM, so the number of images was limited by your video RAM, higher resolution images taking more space.
I needed to show 30 to 40 slides, so I wrote a simple Lua script that loads images one at a time into an Image source, using Forward and Back hotkeys to change slides.
https://github.com/OldBaldGeek/OBS-old-bald-scripts/blob/main/SimpleSlides.lua