March/April Itinerary advise 6 days

You aren’t going to do much site seeing in Nafplio after a flight and a bus ride.

You really have no time to see Nafplio. The fortress is magnificent and we spent a morning seeing it (it is up 999 steps!) I would not go Hydra. You can easily occupy yourselves in Nafplio and surroundings. We spent 4 nights in Nafplio with our young adult children.

I also would do Epidaurus and Mycenae on different days. You have a 14 year old with you. My kids liked Epidaurus better than Mycenae.

One site that is near Mycenae that my kids loved is Nemea. Nemea is one of the four sites that the games rotated between (Olympia being another). There is the original track where you can race even and a really nice museum. They have reconstructed parts of it so more to see than Mycanae. It was a favorite for my group.

I agree with BethFl, on March 30, unless you land very early in Athens the "Nafplio site seeing" will be reduced to a minimum if you take the bus.
The departure bus station (Kifissou) to go to Nafplio is in the city center, almost 1 hour from the airport.
March 30 will be a Sunday, bus service is limited to one bus approximately every 90 minutes.

not sure it is possible to pick up car in Nafplio and drop off in
Ermioni

You can forget this idea.

We do arrive around 8:30 am in the 30th. But I appreciate the advise. Maybe best to leave out Hydra. Would it be reasonable to get a taxi from the airport to the bus station? Any other must sees near Nafplio?

To go by bus to Nafplio from Athens airport:

2 ways to go to Kifissos bus station in Athens

1 -Taxi

2 - Express bus X93

https://www.athensairportbus.com/en/timetable/x93airporttokifisos.html

Location of the bus station in Athens:

https://maps.app.goo.gl/aXpsCALhxBqNjr5z6

Bus tickets and timetables:

https://ktelbus.gr/arg/ticketweb/