There's a 14 minute clip of Bonbi talking about "the thing". I watched it and my guess is that it's something medical. Like some kind of procedure done to her. She's nervous, but she's excited-nervous. I don't think she would be like that if her parents were kicking her out. She also says things like "I won't be able to sleep the night before" and "I'm gonna scream right before it happens", implying it's a singular event, something is done to her and then it's over. Overall she gives off the exact vibe a person before some medical procedure would give.

My main guesses are piercing/tat. More crazy guess is some kind of cosmetic surgery. She's small and has big boobs. She complained about them in a lot of streams, it's probably causing her back pains and we know she likes to cosplay boys. I wouldn't be exactly surprised if she's getting a breast reduction surgery. It also lines up with other things she said like "the thing is an opposite of onlyfans" (smaller boobs -> coomers btfo) and "the thing changes you, changes what people think about you" (surgery like that is obviously a drastic change).

The most extreme guess would be she's transitioning to male, but that's highly unlikely given her parents are hard conservatives and didn't even allow her to get jabbed.