You should check with your carrier in Greece and make sure they are eSim providers.
Also be aware, your phone will have no warranty in Greece. Any savings you might see from buying a phone in the US would quickly evaporate and then some should you need warranty service as it would require you to hop on a plane and come back to the US.
Also, if you do intend to buy your phone in the US, ONLY buy from Apple directly as only Apple sells fully carrier unlocked phones.
We ALWAYS recommend you buy your phone in the country you live and will use it.
Buyer beware and understand the issues you may experience.