If the 800,000thb was never taxed in any country (income - not old already taxed savings) and you stay >180 days in Thailand the year you brought it in Thailand, yes you will have to pay thai tax on it.
"Taxpayers are classified into “resident” and “non-resident”. “Resident” means any person residing in Thailand for a period or periods aggregating more than 180 days in any tax (calendar) year. A resident of Thailand is liable to pay tax on income from sources in Thailand as well as on the portion of income from foreign sources that is brought into Thailand. A non-resident is, however, subject to tax only on income from sources in Thailand."
go siem reap by land (poi pet) or air and Come back by air 1 day later with visa exemption tourist then visa extension 30d if your nationality allow it