It's a problem which is common in the transport sector to decide on where to build warehouses. Unfortunately I don't have recommendations that are open to use, most seem to be custom in-house solutions.

A big question is what you are optimising for? If it's average time, it could be that it is far for everyone. Or are you employees living in clusters that can carry a heigher weight? You already talk about different staff carrying a different weight. etc. etc.

It's typically not even easy to properly define the problem :-)