If it is physically feasible, which it is given we have immune cells and they are just evolution-created versions of these nanobots, then it is just an engineering problem and we humans are pretty good at engineering things.
Things will keep moving forward; macro approaches such as your suggestions will work insofar as we can find the cells. In the end it will probably be a mixture of re-engineering our immune cells to better identify and protect against cancer, macro approaches such as the ones you mentioned, and more.
It is a hard problem, of course, otherwise we'd have it solved by now, but we've solved hard problems before. And with AI it will be easier and easier to do so.