Unless there is something specific and unusual about Lightroom files that would preclude them from simply being copied from one drive to another, then the easiest way to move your files will be to drag them from their location on drive A into the destination folder on drive B.
Please see: Organize files in folders on Mac - Apple Support
The total number of files you want to move may add complexity, but the drag-n-drop method is dead simple.
Another great way to transfer all of your user files and folders would be to use Time Machine app on the old computer to create a backup that you can import onto your new Mac at setup time, of afterward using the Migration Assistant app on the new Mac.
Please see: Back up your Mac with Time Machine - Apple Support
and: Move content to a new Mac - Apple Support
If all else fails, contact Adobe Photoshop Lightroom Support.