Presuming that we could build on what we learned from Ingenuity, and we built, launched, and landed a copter platform that was designed to autonomously map the surface of Mars, what challenges would it be worth undertaking?
You are asking for far too much, and because you are asking for far too much, you are missing the point of Ingenuity, and its follow-on helicopters, which will be better in terms of reliability and in terms of mapping.
Look to our own planet. The Earth has multiple Landsat satellites. Landsat imagery was perceived as very high resolution 49 years ago, when the first Landsat satellite was launched. Landsat imagery is no longer perceived as high resolution. We have several dozen military-grade and commercial-grade satellites orbiting the Earth that make Landsat imagery look like a child's crayon drawings. We have thousands upon thousands of airplanes, helicopters, and drones whose aerial photographs make even the highest resolution imagery from modern Earth observing satellites also look like a child's crayon drawings. Yet there remain vast expanses of the Earth's surface that are mapped at Landsat level accuracy.
We have not come anywhere close to mapping the surface of the Earth using aerial photography. That said, aerial photography has helped a lot, a whole lot. But it is not complete. Asking for it to be complete on Mars is asking far too much.
What will be helpful is to have helicopters do a better job of identifying objects of interest and a better job of mapping the way forward for Mars rovers than those objects of interest / paths toward those objects of interest identified by imagery from the rover itself or from satellites. Think of all the times a Mars rover has had to backtrack because satellite and rover imagery did not see obstacles. Think of all the times a Mars rover has investigated what appeared to be an object of interest that turned out to be an object of no interest. There were also probably times a Mars rover did not investigate what should have been an object of interest that rover and satellite imagery missed.
Better identification of paths and better identification of objects of interest are how a Mars helicopter can be beneficial. Complete mapping of Mars is not needed.