Is refraining from visiting one's parents, and only visiting them once a year due to work circumstances, considered a transgression against them?

It is not possible to define a specific criterion for disrespecting parents; rather, it depends on the customs of the land and the parents themselves. Disrespect occurs by disobeying their commands and not obeying them in what is right and lawful. If your distance from them harms them, then it is a form of disrespect. However, if they are content with it for your benefit or their gain, then there is no harm, provided you continue to honor them, visit them, and make them feel loved. One must be keen to be close to them and do what pleases them, as long as it does not involve disobedience to Allah.