I need it to be like this:

Yate can load subfolders. Read up on the Open Mode. The Open Mode can be on the toolbar and is on the File Menu.

What you're asking for can be done. The files can be moved up one folder. The easiest way would be to load all the folders (Decca Sound) which would get all the files in. (Again see the Open Mode. You want Process Subfolders enabled). You'd then need an action to move the tracks up one folder deleting the then empty folders. This is a one line action and it would do the entire box set at once.

Go to the Action Manager and create a new action. Add a Move statement. Configure the Move statement to be exactly as in the screenshot. You'll only have to set to items: Relative to 1 and enabled the Delete empty folder checkbox. Save the action.

Enable Process Subfolders in the Open Mode. Drag all the Decca folders from the finder into Yate. All files in the dragged folder and any subfolders will be loaded. (It's a good idea to turn off Process Subfolders when you no longer want it).

Run the action you just created. Remember .... all loaded files will be moved up one folder so don't have anything loaded where you don't want it done.