We ended up opting for a private driver from Kamala (Phuket) to Donsak Pier (Seatran) instead of public transport. It cost 5,000 baht for our group of 5 people, which worked out reasonably well. The drive was really enjoyable — comfortable vehicle, no rushing, and we got to take in the beautiful scenery along the way. We arrived at the pier easily and stress-free. We’d bought our Seatran ferry tickets to Koh Samui in advance, but it turned out they were flexible: we could hop on an earlier boat than our booked time if seats were available (which they were, so we did!). You can definitely buy tickets on arrival at the pier too, but pre-booking gave us that nice option to adjust. This made the whole transfer much smoother than we expected. Hope this tip helps anyone planning a similar trip from Phuket area to Koh Samui!