How do I set up the last photo on photos for mac slideshow to display longer?

Which theme are you using for the slideshow? The Ken Burns theme seems to be the only theme with an option "Play Selected for ... seconds", where you can select a slide and adjust time it is displayed.

The vintage Prints theme has no customization option for the duration of the individual slides at all.

You could duplicate the last slide several times and add the copies, but it would not be the same as extending the duration of the last slide. Or create a video of the last slide - from a slideshow with only this slide, with a long duration. Add this video as the last slide.