I arrived on a earlier flight number into Phuket on the same date on my Thai Pass, had no worries. Singapore Airlines said they were told for the Thai Pass you can arrive on the same day or upto 72 hours after the original flight time without any dramas.
Book the SHA Plus+ hotel and then the two tests via PSAS.
Make sure the image you upload has both the hotel and PCR test receipts. I screenshoted each receipt and used MS Paint on my computer to put them in the same image.
I didn't get transport via my hotel, I just got a taxi from the taxi counter at the testing exit. I was under the test and go program, so I am not sure if sandbox requires pre-booked transport. If you want to be on the safe side, organise it through your hotel.