Since you're guaranteed the next strike after 66P in an opponent's back, there's a lot of various setups and juggles, all of which have varying degrees of dmg. What I considered one of the best setups in Vanilla no longer works in Ultimate, so I had hit the lab to find a replacement.
Despite there being multiple nice setups, I personally am the type of player that only looks for, and uses, the most powerful setups. I don't care about setups from the same situation that result in 10-20 dmg less. Of course, "most powerful" is subjective based on each individual's own style for playing. However, of all the setups I'm familiar with, the following two are what I consider the best.
After 66P
1. 9PP2K > sitdown > Must hold
-No part of the 9PP2K can be held. A 7K launcher is available after the sitdown since nothing can be SE'd. A CB is also there, but the timing of 7P is very strict and changes depending on how much (or if at all) the opponent is SE'ing. If the timing of 7P is off, you'll just knock the opponent away. This is due to the sitdown originating from the opponent's back. It's also very easy hold bait when the opponent knows they have no other choice. That, in turn, gives you more flexibility for a CB.
2. 6PP > launcher > juggle
-Can't be SE'd or held. Best option for "guaranteed" dmg. No other guaranteed setup will lead to higher dmg output. Just make sure you're not doing the 6P6P input, which can be SE'd or held, because that leads to a different stun than 6PP.
Here's a video I slapped together showing the above setups. I didn't finish the juggles so I could fit them all in one round.
That setup can't be SE'd. The 66P in the back is something like a 25 frame stun on fastest SE. 1K is 18 or 19 frames. Under silly conditions, the P can be held. But it's literally closer to being a fluke than a real defensive tactic by the opponent.
Also, I've been using that 1KKP setup since Vanilla and had learned it on my own, so it's possible you saw me using it. I showed that setup to a few other Kokoro players in Ultimate.