Paet of the trick is to make it clear and obvious for Immigration, so I'd wait until your final monthly payment is in place and have one single standing order to capture the full amount then
You have been entering on a Tourist visa whilst not fulfilling the criteria as a tourist so you can expect to be questioned. Good that you are going on a more appropriate visa.
Your bigger problem might be getting a flight to Cambodia if your destination is Siem Reap- as of yesterday the Cambodian government prohibited these from Thailand as part of the ongoing spat.