What do you guys do after you knock them down?
What most characters have to do. Back up and allow the situation to reset because wake up kicks are still causing a dumbass stun that they shouldn't be causing. The ground game is gone so Kasumi in particular has nothing to press with besides her launching kick holds and her 9P or 9P+K in hopes to get a BT whiff punish on a wake up kick. But you can't or at least you don't want to 100% rely on those two tactics.
The vacuum holds on wake up kicks is still in the game. So you can try that. Stay just outside of the range on them and guess which option their going to do. If you guess right you get hold. If you guess wrong the kick won't hit you. Knocking someone down depending on the character really doesn't pay all to well in this game.
It stops your offense and unless you have guaranteed ground grabs or attacks. You are forced to reset the situation every time after a knockdown.
In strong competitive play her lows are nonexistent. In doa5 you keep pressure up with Kasumi with 6PK and the rest of her initial poking moves: her jab (punch), 4p this punch had some range to it and its still fast, a frame slower than what it use to be. 2P is her only safe low and she is at -1 on hit so you can't really attack after it unless you're up against someone super slow. 2K is another one but at range, this low kick does not have crouching properties. So poke with it at max range.
Kasumi and her speed is her pressure. You have to stun players to really put pressure on with her. If they are blocking everything then you can't really pressure anyone with her. So you start punch grabbing, once they've stopped blocking. Then shit will go in her favor because unless its Pai or Kasumi the character is screwed trying to attack to while she's attacking. Because this game puts you at a disadvantage constantly for putting on an offense on a defensive player. You have to find ways to open people up because you can't afford to keep being blocked all damn day.
I agree her lows aren't all that great but I never thought too highly of them in first place. I also never used them to keep up pressure with her because of how well good players reacted to them. The one you're using in that string is by far the best one to use by her.
Edit: 66P is another good move but its really a whiff punisher. So it's best if it's used at range to stun someone and then move in on them. She also has the follow up that most forget or don't know about that guarantees teleport attacking moves. But both punches are unsafe. So you can't keep them trowing them out. Her 3K is also good to poke with to get a stun but this too needs some space to be done.
When you get a stun with her which isn't hard to do. That's when your pressure game starts.