Have I committed a sin by leaving my sick parents to work, which led to their death shortly thereafter, even though I was obedient to them and they were pleased with me, and do I owe expiation for that?

If you are obedient to your parents and they are pleased with you, and your travel does not entail neglecting their rights, then your travel is not an act of disobedience. A person's feeling of falling short in fulfilling their parents' rights is a sign of sincerity. And if you fall short in some matters, then that is a cause for Allah's forgiveness, for He, the Exalted, says: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the often returning [to Him]." Al-Qurtubi clarified that the verse means Allah forgives fleeting slips if the person is sincere in their intention of righteousness. Strive to be dutiful to them. Dutifulness can be made up for after their death through supplication, charity, maintaining kinship ties, and honoring their friends.