From the moment you complete your home ownership papers are the only ones you should use- go to Immigration to file your new address as soon as its yours - even if you dont yet live there
If you actually purchase a visa with multiple entry, rather than rely on multiple exemptions, any issues with Immigration should be negated. They just want people to pay their way when they stay longer term.
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.