Just found that on a Mac you can create a slideshow that is not a memory.
Create a slideshow project
There is a lot more control and you can adjust duration.
The resulting slideshow does not appear in memories but in a new "Projets" category.
You can export the slideshow as a movie.
So far I have not found the same thing on the Iphone or Ipad but the exported file can be played on those.
It is far from perfect because it generate a separate file.
I would have prefered rich slideshow feature in the Photo app that work on all Apple devices.
Don't forget to submit feature request to bring back more Slideshow/Memories control on IOS
Feedback - Photos - Apple