The ferry is a much superior experience to flying - in terms of comfort, sights, enjoyment.
In my experience, 1st class was not expensive (less expensive than flying) and even offers one a private berth, but one has to ask for the latter.
I remember the ferry stopping at Mykononos and other islands before arriving at Piraeus. Takes a good part of the day - but consider it a one-day cruise.
All the stops are thrilling - the beauty of all the ports, the fluid chaos, the inherent excitement.
I used to book only a few days in advance, once I get to Athens, and that was in peak summer season. Others with more recent experience may want to comment on this.
The timetable, once I was able to find it, was reliable.
However, once we ran smack into the infamous meltemi wind, and even the sailors were sick. The big ferry was not exactly buffeted around, but there were some moments of balance upset issues...
Maybe I shouldn't have mentioned it. Worries about the weather are so futile. Weather happens. Besides, the meltemi is an August thing. Warren Beatty once said: jealousy is the seasickness of emotions; you think you're going to die, and others think it's funny.