Your argument about unfeasibility of aging damage repair reminds me of arguments of people avoiding to which a human body would be torn apart if humans would travel by train into tunnel at the "staggering" speed of 30 km/h. They had basically well sounded arguments that time. Then people have tried it and find it working 🤷🏼♂️
Yes, repairing all damage caused by aging is very, very hard. But it is neither needed to be done completely to have some meaningful effect (partial repair provides partial life extension), nor we need to work by nanobots on DNA of every cell (in many cases it is sufficient to kill old cell and provide a new one).
It will take some time and a lot of effort. But for meaningful results we do not need such sci-fi tech you have described.