To maintain VAT-Free status a boat has to leave once every 18 months the Customs zone of the Union which is slightly different to the Schengen zone. You could go to Norway for example – Schengen but not Customs.
During the 18 month period of Temporary Admission in the EU Customs zone, there must be a non-EU resident skipper on board or the boat becomes liable for VAT. The EU legislation says the skipper must be “not habitually resident in the EU”. It seems most countries interpret this as “not a tax resident”, which in many countries is dumbed-down to <183 days in-country.
You could spend forever researching tax law and definitions of “domicile”, “habitual”, “customary”, and “tax” residency blah blah blah. Passports and actual nationality seem only passingly relevant. “Centre of financial interests” on the other hand is usually a direct test of tax-residency.
For a Greek “Golden Visa”, you must have financial interests in Greece including bank accounts and an investment; and being there > 6 months would make you indisputably a tax resident of Greece with all the nasty consequences. I’m not a tax lawyer, and this is Facebook. Perhaps others have different advice. My suggestion – see an international tax lawyer. FWIW.