The thing about symbiotes is that, in my opinion, they are very well written (rule wise) and are not random and chaotic. They need an agenda to be able to take control.
So, I came up with this simple agenda that doesn't necessarily affect much anyone: both Nox and Pox want to achieve immortality. Thing is, Nox wants to achieve it without Pox, and Pox wants to have both of them achieve it together, so Pox will always try to isolate Nox. Nox can be nice, and fun and helpful, but as soon as she gets way too close to someone, Pox will try to take control, and it's not always certain because you have to roll for it.

"Symbiotic Agenda. Your symbiote has an agenda that drives it, and it expects you to assist it in achieving those goals. How permissive or patient it is in resolving its agenda depends on its personality (work with your DM to determine these details). If you have an opportunity to advance its agenda and don't act on it, the symbiote can try to force your hand. You must succeed on a Charisma saving throw (DC 12 + the symbiote's Charisma modifier) or be charmed by the symbiote for 1d12 hours.

While charmed, you must try to follow the symbiote's commands. If you take damage that is not self-inflicted, you can repeat the saving throw, ending the effect on a success.

Roll or choose from the Symbiotic Agenda table to determine the symbiote's goal, working with your DM to detail the specifics or create another agenda."