There are more than a couple decent options in terms of chat apps that are end-to-end encrypted. If you're worried about someone getting a hold of your phone—I have no idea about this on iPhone, but on Android—you can set up multiple accounts, so you could easily just set up another account that's for sexting and nothing else and set a different pin for it. More importantly than anything technical, frame all of your nudes, and phrase everything you say, and whatnot, in such a way that even if your nudes and/or screenshots of your chats were to get leaked without your permission, nothing about that would lead directly to you. Unless and until you've already been talking to someone for long enough for them to have earned some amount of trust on your part, and preferably developed a genuine friendship with them that is beyond purely sexual, don't reveal too much about your real world identity, whatever you do; that's how most people end up blowing it if they're going to. Hackers who are good enough that the feds would never catch them by means of anything technical, have been caught by the feds because they got sloppy and then ran their mouths in that order, for example. Same theory applies to people like you and me.
Tbh, if you're being that careful, that's about all you can do if you're asking me.