gaining storage space on my macOS Catalina iMac

If you're sure that the new Photos library has all of the photos that were in the iPhoto library you can delete the iPhoto library.

Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

However, you won't gain the full 414GB of space back. Photos and iPhoto share the same space for the original files that were in the iPhoto library.

So when deleting the iPhoto library you only may gain 20GB of space on the drive, maybe less.

Yes. If Photos created a new library out of your iPhoto Library, and you see all the images there, you can delete the iPhoto Library. Note that you won't gain as much free space as it seems since Photos actually shares many of the images in the iPhoto Library.

If you delete the movies from your Mac, the backup will be your only copy of the videos and no longer be a backup. You should then make a new backup of the movies on another volume, so you will still have original copies plus a backup for safety.

You can, I would just use the finder to go to the pictures folder, find the iPhoto Library and drag it to the trash. Then empty the trash.

Go to your Pictures folder, click on and drag the iPhoto.migratedphotolibrary file to the Trash bin and empty it.

If the movies are on external drives, then deleting them won't make space on your Mac...

IN any event, deletion is the same: drag whatever it is to the trash and empty it.