My guess is that it has to do with Ami killing herself at the tree. I mean, why would she even go there and have a showdown with the entire family (and Nodoka for whatever reason?), win, then hang herself?
My guess is that Ami isn't there for any Tsukioka, but for Nodoka, who've just asked Touka about the sakura tree that never dies. This headcanon of mine is that Nodoka went there to get to the bottom of that tree (both literally and figuratively), and Ami was there to stop that from happening.
Like you said, Ami almost didn't have any strong enough reason to kill those Tsukiokas (during a reset, no less. I'd be dodging Shi-chan at all costs instead of taking my time sawing a rich kid) unless I force the weird presence of Nodoka and that particular room into her incentive. Of course, I still can't explain how Ami knew Nodoka headed that way, or why somehow Nodoka wasn't the first to get dissected, or how Nodoka wanting to know more really contributes to painting herself as Ami's target ; this headcanon simply goes for what is spicier and not what is more possible for now.
As for Ami, she probably went to confront Touka, after what she said during the Dorm Wars. She did have that smile with murderous intent behind it, after Touka's set. How she managed to pack up the Tsukioka bloodline + Yuki and Nodoka is beyond me. Unless there was some sort of chain reaction like: Ami goes to attack Touka, Tsubasa steps in to protect Touka, then Yuki steps in protect Tsubasa, and they all ended up dead in the process. The only reason I can think she would murder Tsukasa would be out of jealousy. I wouldn't put it past Ami for being petty and straight up murdering Tsukasa in cold blood, for the simple fact that she has a mother that's alive (or was at the time) and she doesn't. I mean she shanked Nodoka for looking at Sensei funny. And Nodoka probably watched all that go down and said something, well Nodoka would say and got stabbed, because she's Nodoka.
I might need someone to look into this with me because I didn't think Touka was in the sakura room during this reset.
- When Hima-chan ran through her list, the main cast she was still missing were Ami, Futaba, Nodoka, and Rin. This means Touka must've been accounted for already without visiting the sakura room.
- When Shi-chan went to the sakura room, she counted 5 people and originally I didn't think Ami was one of the five. I thought those five were Touka/Tsubasa/Tsukasa/Yumi/Nodoka while Ami killed the first three. However, with Hima-chan's situation, it feels like the "five grouped up here in the mansion" were actually Ami/Tsubasa/Tsukasa/Yumi/Nodoka.