Here is the criteria: No more 200 deposit transactions per year or less than 2 million baht in total deposit transaction each year, the Thai bank will not report to the dept.
Never bring all money into Thailand. Just a proportion of it enough for a quarter. The bank can give any reason to freeze your money if they think you are laundrying.
The tax policy is truly complicated. Most info available online are false and mixed with truth. If you stay in Thailand for less than 6 months, there is no need to declare for tax.