Pain and suffering are still felt here. Even if it may not affect our original bodies, our current selves are and shouldn't be considered as disposable. Don't forget that while you may not care about yourself, if you turn into a noctaere it could endanger others. Without enough information, I'm not leaving any risks to chance.

[Not that he can fault anyone in thinking that way, but having been around as long as he's had and having to experience that dying, even if it was in a dream... No, it's not pleasant. Sure, if Phantom dies and that's the end of his existence in his world, then...good? Best case??? But if he dies and keeps coming back without any safeguards, then they could have an even larger problem on their hands.]

But the only way would be to look into any kind of sealing or power suppression magic. If your darkness has altered to be more aligned with nightmare energy, then we could look into those avenues as well. This world is filled with different magic and knowledge, both from its own and from other worlds. It might not be easy, but it shouldn't be impossible to think of some alternative, even if temporary.