I've got 25 of 30 so far. But I've just secured the Propontis; will I get a chance to pick up enough stragglers further south or have I just made it impossible practically speaking to complete this objective? Should I let the Persians retake a port?
Yeah, there's no other way to beat this objective unless they can keep building triremes, so you might have to let them take it back a port if you want to complete it. It's not required to beat the game, though.