How do I created a Slideshow in LrC 13.3 on a Mac running Sonoma 14.5

Did I reply to this question in another forum?- For Slideshow order you can do a Custom Sort of photos in a Collection by using Drag&Drop either in a grid view or on the filmstrip, beforer you export to aa video. Once exported as a video the order is set forever in the video.

(Note: You cannot custom sort a folder of images if there is an existing sub-folder.)

Changing random individual slide change intervals  is not available in LrC.  All 'slides' will project for the same set time interval as set in the Playback panel of the SS..

If you add music to a slideshow then the 'TIme on screen' for each photo needs to be set by you to match the 'Time of music'

Example:  Music 3 minutes (180 seconds),  30 Photos,  Set the 'Time on screen' to 6 seconds each photo.

If you have a known number of photos, and a set 'Time on Screen' that you want, then to match music to the length of the SS you may need to edit your music in a sound editor. (Multiple tracks, shortened tracks, etc).

I do not often create Slideshow, so take this with a grain of salt, but a article to look at and a procedure to try.

First the Adobe Document:

https://helpx.adobe.com/lightroom-classic/help/specifying-slide-layout.html

Look at "Rearrange Slide Order"

Next the recommendation.

If you are not already doing this, create a collection for the Slideshow,  populate the collection with the desired photos, arrange them as desired. Create thew Slideshow using the collection as the source. Apparently the Slideshow creation will behave better this way.