Everything aside, all Lex had done was delay the decision about Mary's body, and his secrets. But he genuinely saw no reason to stress himself out about something thousands of years away.
Yes, he definitely should plan long term. But as a result of his own cultivation technique, as well as his system, everything changed too rapidly. Quite frankly, in 3000 years, Lex either saw himself as an extremely powerful cultivator, far beyond what he could currently imagine, or he'd be dead.
He wasn't ignorant of the fact that he quite frequently got mixed up in things way beyond his level. All it would take was one screwup, one mistake, and there would be no recourse. Lex was quite confident in himself, and he always took maximum precautions in everything he did.
