@Collette ********
probably the easiest would be to do test & go, book your day 1 SHA Extra Plus hotel in Bangkok, and then your day 5 hotel where your destination is.
This means you'll get off the plane in Bangkok, go to the hotel and get tested, then once you test negative you'll be let out and can continue to the island.
The reason this is easier is because you only need to purchase a flight to Bangkok and have many options for that.
There are some sealed routes you can do for Sandbox like Koh Samui for example, but you have to buy that all on one ticket to enter Thailand, and there are limited daily sealed route flights so your transit in BKK could end up being 12+ hours.