I found a way to make this much easier without having to change your configuration. i.e., when tap jump is on, L and R both block, Z grabs, and C-stick smashes.
You'll notice that when you hit up first, then hold A to do a Usmash, you sometimes jump. When you hit two buttons, there is a small gap between the first and second inputs, so what that means is the order you hit the buttons in doesn't matter. Therefore, you can hit A and then Up on the control stick to do the U-smash.
Here is a breakdown of what to hit, and when.
Tap Forward to dash.
C-stick Down to begin the attack.
(Wait for attack to begin)
A and then IMMEDIATELY hit up.
So, that's Forward > V-Cstick > A > Up.
It will be a little odd at first. Try to say "One-Two-Three-Four!" in your head. Just make sure you press Up IMMEDIATELY after A and it should work fine. I can get it pretty much every time now. Try to get it in one fluid motion. Happy DACUSing :D