Itinerary for Syros/Serifos

I’ve no experience of using a travel agency, so it might be that this is a reasonable price for the convenience. It looks to me though as if you could do it more cheaply booking direct, but then you would need to sort out your own ferry times and transfers.

One basic question: are you planning for single rooms or to share? If you’re sharing the hotel rates are really high.

On a couple of specifics
a) there is a hotel Arion in Athens but the link is obviously a mistake. This is a more likely link
https://www.arionhotel.gr/
I don’t know the hotel but it’s in a fairly central area.
b) again I don’t know the Dorkas but it’s in a nice position - an easy walk to town and a nice beach.

Do you really want to do two islands? If not you might want to consider Andros: it’s off the beaten track, has good beaches and walking and is an easy trip from the airport. You can easily arrange it yourself. This is their tourist site

https://andros.gr/en/

Alan
PS I’m not trying to put you off Syros or Serifos. We like both of them.

With the information you have given, I redid the calculations (July rates) I find a price of around €1100 for 2 people which includes the ferry tickets and the price of the rooms including breakfast (choosing the cheapest rooms) I have not included transfers between accommodation and ports which can add between €50 and €100 depending on the case.

So €680 per person seems pretty good to me.
The agency has to make a profit to live.

The Arion Hotel is located a little north of the most touristy areas but it has the advantage of being near the Athens Municipal Market which is a must-see.
In addition, less than 50 meters from the hotel, there is the restaurant "To Karamanlidika" where you absolutely have to dine if you like charcuterie

https://karamanlidika.gr/en/

I don't know the Brazzera hotel in Syros

About Dorkas in Serifos :
Booking.com reviews of Dorkas are very good.
I was in Serifos last fall and often went to the area in Livadi where Dorkas is.
I found this place very good,
The "town" of Livadi with all the bars, restaurants along the beach is 200 meters away.

Dorkas is next to Livadakia beach which is not huge but nice, with a family atmosphere in September (probably more lively in July).
You see all the ferries arriving and departing from Serifos pass through the bay and if you are looking for a quieter beach you can take the path that goes south for about 300 meters and arrives at Karavi beach.

Thank you for all the great information. We are still thinking through options. Looks like using travel agency in Athens is not as useful - they don’t use Airbnb’s and their choice of hotel seems unnecessary $$$. Booking ferries feels a bit intimidating to me - we messed up once in the past and it was impossible to fix when we found out (two days to departure).

Does anyone have impressions about
Spetsai, Skopelos and Pelion (peninsula) as potential vacations spots - again in terms of beaches, and being not too tourist-y?

If you are used to planning and booking yourself with Airbnb or Booking or directly with accommodation, you really have no reason to go through a tour operator.
For July the choice will become very limited if you do not start looking for accommodation now

I don't know what kind of problem you had in the past with the ferry. In the Greek islands, apart from cancellation problems due to bad weather (or sometimes strikes), booking and use is very simple. In fact, they are like buses for Greeks who live or work on the islands.

If you want to see ferry lines, timetables and fares, go to:

https://www.ferryhopper.com/en/

or

https://gtp.gr/

The island of Skopelos is in the Sporades, to my knowledge there is no ferry connection to go there from the ports of Athens.

The island of Spetses can be reached by ferry from Piraeus. But it's a very small island where you may be bored after two days, moreover if you want to go to another island of the Cyclades like Serifos, Sifnos, etc. you will have to return to Piraeus to take a ferry.

The Pelion Peninsula is a different kind of adventure. You will need to rent a car.
I do not know this peninsula but I would like to go there one of these days.