"Picture Frame" mode on modern iPad

For the built-in Photos app, you can get subtle effects (dissolve, specifically, is available) in full screen of iOS 11, but I can’t find a way to randomise it, unfortunately.

Since I never used the picture-frame mode on my iPad 1, I can’t picture it (heh) in my head, but if you go to Photos → Your Album and choose Start Slideshow, you can tap on the slideshow and on the bottom right an Options button will be revealed, allowing you to change the speed of the transitions, toggle Repeat , choose or disable an accompanying musical theme, and choose the transition type (which includes Dissolve)

I’m not familiar with other slideshow applications, so I can’t suggest any.