This approach is making perfect the enemy of good. There is no perfect way to solve this problem.
> If we do the same thing with KiwiFarms, we just make it harder to monitor and easier for serial-abusers to collaborate.
It's still a website, it's still accessible. That part hasn't gone away. If the contention is that launching TOR somehow means it's "harder to monitor" then I have some news for you: Lots of monitoring of darkweb stuff goes on every day.