Shinji was somewhat pleased to see a relatively successful experimental subject like Zabuza emerge, but research into "divine techniques" was still in its early stages.
As for Zabuza, Shinji was focused on research and had little interest in cultivating him.
First, Zabuza was unruly and difficult to control. The risks of devoting resources to someone like him far outweighed the benefits.
Second, Otsutsuki Shibai's chakra bore the remnants of his will. Though not to the extent of Karma, it still had a tangible influence on every experimental subject.
To put it plainly, the chakra of every subject who had received Otsutsuki Shibai's cells, including Ada and Daemon, bore Shibai's "anti-counterfeiting label." Their chakra was no longer pure, so aside from continuing to transplant Otsutsuki Shibai's power, they had almost no way to improve their strength.
In other words,
whether it was Ada, Daemon, or Zabuza, their strength had reached its limit unless they underwent another transplant surgery. It was almost impossible for them to grow stronger through training.
Therefore, the experimental subjects like Ada and Daemon, as well as Zabuza, might currently be able to match Karin, Hikari, and Shion, but their potential was nowhere near as great as theirs. That made them less valuable to cultivate.
What Shinji really needed wasn't these experimental subjects themselves. By creating one successful subject after another, he could analyze Otsutsuki Shibai's chakra and uncover the true secrets of "divine techniques."
Once he unraveled the secrets of "divine techniques," Shinji would be able to take his strength even further.
This was important to Shinji, who had already become a Kekkei Mōra and reached the pinnacle of power. It was now extremely difficult for him to get stronger. He had only a handful of options left: master unfamiliar and rare high-level dōjutsu Kekkei Genkai, or