We will leave Hydra on Wednesday, October 29. Ferry schedules are not approved yet for that date, but it looks like we will be able to take one from Hydra to Ermioni that day.
According to Rome2Rio, we can take a bus or a taxi from Ermioni to Nafplio. Naturally, the price difference is significantly lower by bus (16 EUR for both of us) vs. taxi (65 EUR for both of us). The bus takes a slightly different route for part of the journey and also takes about 30 minutes longer.
I have read that you have to change buses along the way and/or that you have to take a taxi to Kranidi to get the bus, but R2R shows the bus station as being a few blocks from the ferry terminal in Ermioni.
This discussion -- https://community.ricksteves.com/travel-forum/to-the-east/easiest-way-hydra-to-nafplio -- is the best I could find on this topic but it is over 2 years old now, and I found no followup. That's why I'm asking the question.
If you have advice about this route from Hydra to Ermioni to Nafplio either by bus or taxi, I would really appreciate it. It seems the most direct, but we are going to be there at the end of October.