Greece Itinerary Input - 12 nights/Solo traveler/November

Nafplio is in the Peloponnese and Delphi is in central Greece, they do not depend on the same bus networks.
There is no direct bus between Nafplio and Delphi, you have to go back through Athens and the journey time will be at least 6 hours depending on the bus schedules.

If you keep the itinerary you have planned the other option is to find an organized tour to Delphi from Nafplio like this one with a departure at 8:30 a.m. and a return to Nafplio late afternoon.

https://www.uniquegreektours.com/en/tour/delphi-tour-from-nafplio/

It's up to you to decide if it's worth it or not.

As a general rule don't try to see too many places relying only on public transport, you will spend more time in buses and bus stations than in the places you want to see

It may not be worth the effort going to Chania (Crete) for just 4 days. You'll lose time getting to and returning to Athens then time getting to Nafplio.

You could spend your entire time in Chania and still not see enough of the area.

Four nights in Nafplio is fine but then you'll have to get to Delphi which will also eat up time.

Three nights in Athens will be good.

I think it may be better to stick to one location plus Athens to make the most of the time you have for Greece.

Relying on the bus will take flexibility and patience as bus schedules may not be convenient.

Renting a car will provide you with more flexibility and freedom.

The Bradt Guide to the Peloponnese is really good and will hopefully help with your time in Nafplio. There may be some day tours you can organize while there if you wish to see Mycenae, etc.
It may be easier to do a day tour of Delphi from Athens, if the transport is not convenient.

In October 2018 I took a similar solo trip. My trip was 14 nights, one night on the plane, 13 nights on the ground. I started in Nafplio (flights from Detroit to Athens, buses to Nafplio), 3 nights in Nafplio. Maybe with a 4th night I could have taken another day trip. I took buses to Epidavros and then Mycenae both the same day. From Mycenae I had to take a taxi from there to the bus in the parking lot of a cafe because the bus doesn't leave from Mycenae in the off season. Then i spent 2 nights on Crete. I suppose I would have been as happy if I had went to Olympia, the most well known of 4 sites of ancient Olympic games, in Western Greece.

In Crete, are you sure you want to spent 4 nights in Chania or would you like to split your time between Heraklion and Chania? I was in both. Heraklion is rather underrated, maybe because I saw a retaining wall instead of a beach, and much of it was destroyed in world war 2. Heraklion has an archaeology museum that might have occupied me for 3 hours, the Knossos Minoan site, a Venetian fortress called Koules that has been marvelously restored and you can pay to go inside, parts of old city walls, a reconstructed Venetian looking town hall, and so on. Check the Rough Guide to Crete, and/or the Lonely Planet guide to Crete. With more time on Crete I would have seen Rethymno, the Samaria gorge, and possibly somewhere else.

I spent t a night in Delphi, the night after my ferry from Chania to Piraeus. A night in Delphi is a good plan. There isn't necessarily anything wrong with going to Delphi after Nafplio. On the way you would just switch buses in Athens. You might have to buy just a ticket to Athens and then in Athens buy another ticket to Delphi. I took overnight ferries from Piraeus to Heraklio. And from Chania to Piraeus. The ferries worked ok. I had a bed a in shared room with 4 beds both ways. One way i had the room to myself. The other way there was just me and a non-english speaking guy who stayed quite the vast majority of the time. Maybe I would have seen better scenery from a plane.

Are you sure 3 nights is enough for Athens?

This looks great. Starting in Chania is absolutely the right move. There are tons of things to do in and around Chania which can be done easily with public transport.
For the rest I would recommend that you get into a car as soon as you land from Crete. The rental cars are feet from the terminal and you'll be on your way to Nafplio or Delphi right away.
What you want to avoid, regardless, is having to make numerous trips from the airport to central Athens and then back. Thus, if you did have a car, you might consider just paying 10 Euros / night and parking it, making it available for the return to the airport that will feature a drop-off feet from the terminal.

Having a car gives you easy access to the Corinth Canal, Acrocorinth, Argos, Epidaurus, Mycenae, Tolos, Nemea, Arachova, Galaxidi, Nafpaktos, Hosious Loukas and much more. It's easy driving, too. Brings Olympia into play, too, if you're interested (personally, I don't bother doing both Olympia and Delphi on the same trip as they are very similar).

My only concern is that November is after tourist season and perhaps not enough will be open in Nafplio to merit 4 nights there. Sunset at the beginning of November is 5.30 pm and days just get shorter after that, so you want to make sure there's enough to enjoy during those long evenings. OTOH Chania is likely to be fairly lively then with a lot of restaurants, museums, and other sights.

Very wise to look at the greatly shortened days ... and off-season also many many historic sites & Museums close at 3 pm - so you need to schedule accordingly, including avoiding driving in the dark. As to worries about Nafplio, I think it will have events going on, since there's a university nearby, and Old Town residents (tho many now are nongreek expats) attend cultural & musical events & dine out ... best mainland option near Athens.