I have same issue with dual citizenship, usually I book the ticket using the name in the passport of my place of residence and just show both passports when immigration or check in is being difficult about it. In this case I would put the name as it is in the Thai passport
Thailand has no exit requirements. So if your airline doesn’t require one and Canada only wants to see an antigen test, then you only need to do an antigen test
My friend and her baby tested positive on arrival last week and their levels were well above 30 (meaning covid detected but not transmissible) and after some back and forth between the bungalow owner and a nurse (on the phone) they were approved and free to leave the room