Slideshows in Photos 5.0

It is not possible to move the text boxes on the text slides. You could just try to use a different theme, where the text boxes are smaller.

Or avoid the text slides and create the text slides yourself.

• Create a duplicate of the photo and write the text into the photo with the markup Photo Editing extension.
• Then use the annotated version of the photo as the text slide. See: Use Markup to write or draw on a photo in Photos on Mac - Apple Support

For example - a photo with a smaller title:

I think, Apple is replacing the slideshows by the Memories. The Memories are getting updates with each Photos updates, but the slideshows have remained the same since the very first version of Photos in 2015. And in Photos 5 on Catalina we cannot even play videos any more in a slideshow, if we are using a more elaborate theme.

I am using the screensaver to play my slideshows. The third-party ArtSaver can automatically show the titles, filenames, etc. as an annotation for the selected photos, so I do not have to add them manually to title slides: http://www.cs.uni-bremen.de/~zach/software/ArtSaver/index.html