Like other SHA+ hotel, they will send you a letter of acceptance that says the duration of stay and pcr-rt test package. You have to prepay by credit card or bank transfer. They weren't very responsive on expedia or other booking platforms but they reply within an hour or two on their business email.
Expedia offered me to update the bkk to hkt for another date, but that change would have cost me 1000$, which was beyond ridiculous for a domestic flight. After insisting they were able to cancel that part without refund and no extra charge.
I have the same itinerary in december and I decided to have the bkk-hkt segment cancelled (so my luggage doesn't end up in phuket) while we need the mandatory 1 day test&go.
Ill be spending a week in bangkok before flying again to phuket since its only 20-50$ cad one way and far less expensive than cancelling and rebooking direct flights to hkt
It's not that complicated to get the thailand pass before travelling. It replaces the certificate of entry and some airlines are making sure you have your Thailand pass before boarding.
tp.consular.go.th
And heres is a recent summary of travel restrictions to thailand I saw in another group: