Creating and saving a simple movie/slideshow from photos with a chosen music soundtrack

I am trying to create, what I believe to be(!), a simple movie/slideshow project and am really struggling with certain aspects. The short-story is - I want to convert a load of photos from 1 album into a short movie, with a certain song playing in the background. The ability to show some of the 'live' photos would also be a nice to have, but not critical.

The issues I've got are:

1) If I create as a slideshow in the Photos app on my iPhone, I can't seem to save it - only view it.

2) If I create using 'Memories' on the iPhone, the Photos are too zoomed in and I can't edit this. I also can only select certain preset music / soundtrack options, and not a track that's in my library.

3) If I use iMovie on my MacBook, it can't find any of my music library "The contents of your Music library will appear here" despite the Music app on my MacBook having the library within it (it's not a copyright issue as I can create a simple test movie using iMovie on my iPhone with my chosen track in the background) and 2 of the 43 photos appear as completely blurry (all photos taken on the same iPhone).

4) If I use iMovie on my iPhone, I can get all photos in with the right soundtrack, but I can't find an option to Crop to Fit the portrait images.

I have tried resolving the iMovie on MacBook issue by restarting, and deleting the app / reinstalling - but no joy.

Really grateful for any advice or suggestions!