you need to have a complete ticket from origin-bkk-cambodia, it should not be separate tickets,,,as bags need to be tagged to final destination and you need to remain in the transit area without going thru any immigration or customs areas.
Unless Thai has totally different rules than USA, then she should be arriving into the Kingdom with the Thai PP for her. I would certainly check with the Thai Embassy on that.
there are a lot of requirements to do before you can even attempt to fly to BKK via the Thailand Pass that must applied for, good thailand pass and go thru that process, which will also require quarantine in BKK as of right now.
all I am saying you need to fly nonstop into the airport in Phuket if you are doing the sandbox,,,you cannot make a connection in BKK to get to Phuket. so you need to find a flight from europe nonstop to Phuket, or any other country but it must be nonstop to Phuket.
meaning you need to fly from a foreign country, you cannot make the connection in BKK, either fly via Europe and straight into phuket or the other direction via an asian country, for example via Singapore..