I use Adobe Premiere Elements. It runs around $75 -, $100 to own. They have automatic assembly as well as pro modes. There's free music on the web but what I do is to buy a specific song I like from Amazon. Then download the MP4 version. It's usually 99 cents up to $1.25 that I put on my slide show.
Regarding your problem of finding things, the trick is to create a single folder and put everything into that one folder including the final created movie. The songs, movie app, stills, etc. That way all the files that makes up the movie stay together. Otherwise, if they're spread all over the hard drive, when you move things around, the movie app won't be able to find the files. Even the songs. What I do is if all my songs are kept in one folder, I'll make a copy of the songs I'm using for the movie and place a duplicate in the special folder for the movie.
Premiere Elements also also will create an archive of all parts of pictures, narration, songs,m etc into one file for saving later if you wish to modify the poriginal.
Here's one where I incorporated slides and short video clips.
https://www.youtube.com/watch?v=mcmwLSiS-as&t=5sHere's another with only stills.
https://www.youtube.com/watch?v=MogdCeRNqBM&t=69sAlso, note that Windows 10 if you use that has a Movie app that's free.