In 2019 I looked into the question of how to a) stay in Greece longer than three months at a time and b) keep sailing my non EU boat during that extended time. I employed a lawyer to do this with the above requirements as her brief. This immigration and maritime (odd combination, I know) lawyer started the process to allow a longer stay in Greece but unfortunately did not take into account the resulting obligations of temporary citizenship. I only discovered the downside during a casual conversation with a very knowledgeable customs
officer on Patmos. I inquired again on Leros with another customs officer whom I also knew to be competent (some, like too many port police officers are not up to date on new(ish) rules and regulations). I put a stop to the lawyers efforts as a result. If I had obtained a temporary residency, I would have been allowed a maximum of 30 days sailing my non EU boat with a discretionary option to add another week. These are the facts on the ground as of 2019.
If I can get back into the EU (Canadians are currently on the wrong side of the covid list at present) in 2021 I may apply for an extension to the standard 90 day visa. I have obtained those twice before through the Greek embassy here in Vancouver
. One can only apply a maximum of 90 days prior to going to Greece however.
I hope this helps with your question. Good luck.