Wow, this has gotta be one of the most powerful shortcuts in Photoshop, it does two clever things (almost 3!) in one keyboard shortcut!
Just use the shortcut Apple +Alt + Shift + E to copy all layers (copy merged) and paste into a new layer, all in one go! This saves selecting all, then copy merged, then paste - so it kinda does 3 things in one go - now thats what you call a clever Photoshop shortcut in my book!
The best part about this is that it respects transparency, just like the ‘copy merged’ command. You should be careful though, as you may have different layer blending options on your layers like multiply, and with this command it merges everything visible into a new layer that is set to normal blending mode, so it may screw up some blending on layers below.