I don't think you have to worry about how drag changes with airspeed - this is just an issue that the aircraft has a maximum airspeed

That's correct. Using some simplifying assumptions one can explore that relationship further. Assuming that the power required to stay airborne is fixed (~ 130 W for a Mavic 2), that drag goes with the square of the airspeed, and that the drag coefficient for the M2 is 0.025, the energy usage per unit distance, as a function of windspeed (headwind) and airspeed can be calculated analytically:

View attachment 79935
The curves show minima, as expected, that increase with increasing windspeed. Differentiating with respect to airspeed to get the location of those minima:

View attachment 79936
So for zero headwind, that indicates an optimum airspeed of 13.8 m/s (31 mph) to maximize flight distance as a function of energy usage. As the headwind increases the optimum airspeed also increases. 21 m/s (46 mph) is around the maximum airspeed of the M2, and so that suggests that it will struggle into a headwind of greater than 10 m/s (22 mph). We know it can go into greater headwinds than that however, indicating that the assumptions are not completely correct. However, the basic trend is correct.