Here's a short report on how I have moved about the neighbouring provinces after finishing the Phuket sandbox, and how the practicalities have been.
My plan to go to Krabi went bust due to the suddenly tightened regulations there, so I opted for Khao Lak in PhangNa instead. No minibusses or other public transport currently as far as I understand (was told so by a couple of places I asked in Phuket, and the terminal in Phuket town center seemed quite dead when I passed it by). So private transport it was, then. Before crossing the bridge from Phuket, there was a separate lane for foreigners and a counter where the officials asked for my destination and checked the certificate from my Phuket hotel for the succesful completion of the 14 days sandbox. There was also a form to be filled with contact details, including the destination in Khao Lak and the reason for the trip. I wrote "Holiday." The stop took just a couple of minutes. I was the only foreigner crossing at the time.
I had only booked a couple of nights in Khao Lak, Bang Niang beach. Extremely quiet. As far as I could see during the two days roaming around, I was the only tourist in the whole area! Nice beaches all to myself, but of course very limited services remaining in the beach area. The tsunami memorial park was sadly in disrepair. If you can, please consider visiting the area to support the few remaining tourist businesses. They really are in desperate need of visitors. A fair amount of stray dogs around the deserted tourist infrastructure with various kinds of attitudes towards a rare bypasser were a bit of a nuisance but not a big deal.
My next destination was Surat Thani town, where I was able to arrange a couple of locals to drive me to for a similar fee that I paid from Phuket to Khao Lak (again, was told that the usual once-an-hour busses along the main road are not operating). My hotel for one night in Surat Thani told me that they are not currently allowed to book ferry trips to Samui, and I should instead directly visit the Seatran ticket office.
The lady at the Seatran ticket counter was by default asking for a 72-day covid test for travelling to Samui. After I insisted that I had only been in orange provinces (see my posting about the current domestic travel rules under the Thailand local / Ko Samui section of the forums), she took photos of my passport, Phuket sandbox certificate and vaccination certificate and asked me to come back at 10AM for getting the ticket for the 10:30 departure. I don't think she had seen an European Union vaccination certificate before, as she at first mistook it for an outdated covid test certificate. For the ticket to be issued, I also had to register online at https://healthpass.smartsamui.com and fill information about recent whereabout and potential symptoms to get the Samui health pass QR code. This was also later checked both when boarding the ferry and when disembarking in Samui.
In sum, travelling around the neighbouring orange provinces (aside from Krabi) after the sandbox is possible without any further covid tests, although many of the cheaper transport options usually available for tourists are not currently operating. Returning to Phuket will require at least an antigen test. I plan to have a PCR tests shortly before my Samui-Phuket flight. The same test will still be valid for my return flights to Europe a day after.