If immortality is the only goal, he already has it if he just closes away in the pocket dimension. Obviously he misses the sun and wants to return to the daytime earth, perhaps ruling it too, otherwise his whole 'making subservient demons' thing doesn't make any sense, neither does searching for the 'cure'. Having time means he can wait for this unique group of Hashiras capable of killing uppers to wither/die out while Nezuko becomes alone in the world as she ceases to age. Being able to draw threats anywhere into this dimension means he can choose when and where to engage, effectively, putting his forces to the best use with the least risk. I don't buy that he didn't consider that with his time and his paranoia... even if he sees the Hashira as below him, he has experienced fear of death before from the earring swordsman, so simple ignorance won't cut it for an excuse, either.
Because of all of that, right now, that whole climactic scene is unprecedented and silly if there is no catch or drawback; I don't want to headcannon it because I think it is really lazy/rushed writing as is. I hope the power turns out to be another demon's who had to do prep work or meet a condition for this to happen. Just a pet peeve of mine.
Any yeah, not targeting Nezuko with it is absolutely bonkers.