28 day itinerary in Greece

Saving Athens for the end, and the flight home, worked well for us. Based on our all-Crete trip the following year, I’d add a day for Iraklio (aka Heraklion). If you’ve just got the 4 days for Crete, that would make it 2 for each city, but I’m glad we had 2 days in Iraklio.

As for Delphi - the ancient site is amazing, and it’s good to be close by. But we’d reserved 2 nights in modern Delphi, and the first was so noisy, we relocated a few miles away to Galaxidi, much more serene, and a far better place for us to stay. This was in April 2018. It was warm enough that we needed open windows to sleep, but the partying down the street in small Delphi was too boisterous. Maybe these were students on Spring Break; maybe May will be different; I don’t know. But Galaxidi was a superior experience, and still pretty close to the ancient Delphi.

The locations certainly look reasonable for a 3-week-plus trip.

I think you could take a day off Delphi and add it to Athens. If one of your three days includes driving from your prior stop, that gives you 2.5 days max for a city that is incredibly rich in history.
I haven’t been to Paros or Naxos, so I can’t speak from experience; but you are giving them, together, way more time than Athens! Do they warrant so much time?
Also, you really do have a lot of destinations. I think you may get quite tired of the packing up/checking out /traveling/checking in routine. Not to mention the time eaten up by doing that.

Your schedule is a bit ambitious in terms of places to see.

1/ You are going to spend 2 1/2 days in Crete which is an island that requires at least a week to see only half of it.

2/ Note that on May 6 you will only have one ferry to go to Santorini. The Seajets from 8:00 a.m. (98€/person)

3/ Wanting to visit 4 islands including Crete in just 12 days is not a good idea. You will spend a lot of time packing, accommodation-to-port transfers, ferry waiting and ferry crossings, etc.

The more crossings you make from one island to another the more you take the risk of encountering a ferry delay or cancellation which can alter your entire schedule.

For the Islands part it would be wiser to remove Crete which you will not be able to enjoy in 2 1/2 days and to keep 2 or 3 nights in Santorini (fly to Santorini on May 2) + 2 other islands maximum.

Another example of your misjudgment of time spent at each location:

On May 15 you will not have a morning ferry to return to Piraeus. Which means that you will be at Piraeus port, if you are lucky, at the earliest around 3:00 p.m. and at Nafplio around 6:00 p.m.if all goes well.
So when you estimate your time spent in Nafplio at 3 days, it's actually only 2 full days. May 16 and 17.

Is it safe to assume that you mean May of 2024? If not, then accommodations for this May will be very limited. I love to have an active vacation, but IMO your itinerary involves too much packing/unpacking/traveling to the next destination. You will have lost a large chunk of the time available to enjoy a place. For example, by the time you ferry to Santorini, and check in to your hotel then you have less than a half of a day left to explore. That leaves you one full day and then it's back to packing to leave the next morning. For starters, I would pare down the days spent in Delphi. Heraklion was not a favorite of our trip, but I'm guessing you chose that because of the ferry, At the most, I would choose two islands. As mentioned, all of the ferry excursions have the real possibility of being problematic if there are rough seas, or a strike on the day of your travel. (firsthand experience with that and it was a real headache) We found it very efficient to fly from Naxos to Athens. You have very little actual time to spend in Athens, and there is so much to see there. It can be difficult to do, but my philosophy is to approach a trip with the idea that we will not be able to see and do everything that we want, so take a good hard look and prioritize the places that are most important.