Szayel has all the tools he needs to sabotage Nemo right now. Being healthy again is making her less vulnerable, not more.
On the contrary, the only thing scarier than a Szayel trying to hurt you is a Szayel who earnestly thinks he's
helping. And beyond him installing fun but dumb stuff like acid spit or cool combat claws or what have you the concern is that, like...he'll install things designed to not just kill people but hurt them, cripple them, take them apart. Hollows aren't exactly
nice but Nemo pretty reliably tries to make things fast and relatively clean, she's not a sadist or a pursuit predator, she isn't interested in dredging things out on multiple levels and, iirc, is often unsettled at the emotional trauma her abilities can cause. Pairing those with abilities designed, first and foremost, to inflict
physical trauma is only compounding the problem imo. And Nemo's forced into dire enough straights often enough that she'll be sorely tempted to use them and ngl I don't
especially want to read an update where Nemo graphically dismembers a dude with her ravenous, corrosive, retractable mandibles or w/e.
She's worked really hard to get to the kind of spot, emotionally and...dunno spiritually? Where she is right now. Running the risk of backsliding 'cause she got monstery grafts doesn't exactly feel ideal.
Beyond that I'm not super fond of the set up and what it says. Nemo's passing by all these tools and technologies made to basically torture Arrancar and Hollows, to tear them apart and piece them together, to do, in a sense, what Aizen does just much more brutally and viscerally. She knows that the guy who runs this place uses them regularly, and she's deciding that she wants a piece of that. She's, on some level,
agreeing with Szayel when he goes "yes, you need fucked up mad science to perfect your body and unlock your true potential". It's kind of callous in a way? And it's heavily emphasizing the fears of Nemo. The fear of being weak and being picked off. The fear of being useless. The fear of being hurt again.
The Nothing option feels much more like an emphasis and affirmation of her current path in contrast. Where she's still afraid yeah but she's not ruled by it. Where she's able to prioritize emotional and moral considerations over purely physical, immediate concerns. Where she's able to decide something's wrong and she's going to
do something about it.
I don't really grok the "if you're making a deal with the devil just be as selfish as possible" argument frankly. It's kinda mercenary, cold and calculating and maximizing, and that's not something I really strongly associate with Nemo?
Edit: Also big ghost moth flying out into the night with yet another samurai-scientist's research notes is a fun thing really.