While I agree with you that space to land is an issue, in terms of trucking in packages to smaller depots, Amazon already does this in some markets like the UK, where it is investing heavily in regional depots and doing "first level" delivery from their large warehouses to the smaller depots themselves.

Here it depends on quite high population density in the regions they are putting depots, which still has the landing space issue, but e.g. their two London depots puts them within possible delivery distance of several million people with front or back gardens. A large proportion of the ones who don't have private gardens have communal spaces.

The bigger problem than where to land for me would be how to handle "handover" to their customer without ending up leaving packages in plain sight in areas where it's easy for someone to snatch it.