Yeah, the giantess in this world are still basically human somehow, but they have to adapt to their giant reality. For example, they consume more food than before so they have to change the way they produce it. All giantess are split into a small community of 6 to 20, each of them produce different food and can trade them at a central market hub that connect all different community nearby. The main goal of this is to avoid long travel and food waste as much as possible. That concept isn't perfect, but efficient enough to make sure all giantess could eat correctly.
Another adaptation for the giantess is they are simply too heavy to use any kind of transport such as cars or planes so they have to travel by foot only. Trading with other giantess living on another continent over sea is nearly impossible. They can still use a boat to access other continents, but this means of transport is very slow and pretty dangerous for them. Since the giantess eat a lot, they have to carefully manage their ration during their travel; if not, it's a certain death.
For the harem path, it will consist only the six giantess the MC interact with in the main story because let's be honest, the MC won't live long enough to have sex with all existing giantess which is estimated around a million.
And finally, I like your suggestion to make the MC as an ambassador between the to world. I will keep that in mind.