slideshow using Photos for Mac

Try it this way: How to turn an Album into a Slideshow and reserve the sorting - Apple Community

In Photos 3.0 on High Sierra and Photos 4.0 on Mojave:

When you use the file menu to create a slideshow from selected photos, the resulting slideshow may have a different sorting.

Try to create the slideshow from the contextual menu instead of from the file menu.

• Select the sorted album with the photos in the sidebar.
• Ctrl-click or right click or tap the album with two fingers.
• Now select "Create > Slideshow"

For some reason this will preserve the order (at least for my albums in Photos 3.0) while using the file menu will not (the screenshot below is from Photos 3.0).

It is not easy to rearrange the photos once they are in the strip with the slides. The more elaborate slideshow themes have a fixed order of templates, and Photos wants to select the photos in a way best fitting for the predefined templates. I am only using the slideshow themes with just one photo per slide, Ken Burns and Classic. That has the added benefit, that the animations will not distract the audience from watching my photos.

I found this solutions in one of the discussions here, posted by the user pawey47 :

Re: How can I sort slideshow photos chronologically?