You could turn on flat view (mixed, no folders), then select all and drag to the file display background to move everything to the main folder. That's what I tend to do.
Or put my old DirFlatten tool on a button:
cd {sourcepath$}
"C:\Path\To\DirFlatten.exe"
Or there's an Opus-only solution from Move all subfolder files to current folder - #9 by MNeMiC which essentially automates my first suggestion:
SET flatview=MixedNoFolders
SELECT ALLFILES
COPY MOVE HERE FLATVIEWCOPY=single
SET flatview=off
(I haven't tried that but it looks like it should work.)
Of course, be careful and try things out in a test folder a few levels deep in case it doesn't work like you expect.