I suspect other companies haven't loaded their summer schedules yet, a common occurrence in Greece.

Here are two sites I use for greek ferry schedules, one company-specific, one agnostic and in my experience unbiased.

Blue Star is a good company with modern ships and moderate rates, a good compromise compared to slightly cheaper old boats and often substantially more expensive high-speed boats.

However, I always make it a point of leaving one buffer day between arrival in Piraeus by ferry and departure elsewhere, as ferries are at times delayed or canceled due to rough seas. Given your tight schedule, I'd suggest considering flying from Santorini to ATH on the return leg, which could buy you a safe extra day in Santorini (Aegean Airlines would be the preferred alternative). The transfer from the airport to your cruise ship might actually be smoother than trying transfer ships in the sprawling zoo that is Piraeus.