I'm curious, what is harder to implement: autoland for airplanes, or autoland for rockets (spaceX)?

I don't know if these are comparable.

But two interesting data points from the Wikipedia article I linked are that the first aircraft certification for ILS Cat III was in 1968, and Cat IIIB in 1975.

And IIRC by the 1980s, autoland was already a pretty common feature.