Under 65 a person can earn 210,000 a year before tax is due on income (60,000 personal allowance plus the first 150,000 is tax free).
Even in Bangkok people doing ordinary jobs don't earn enough to pay tax. THB 10,000 -12,500 a month (120,000- 150,000 a year) is the sort of wage you'd be paid in farang-facing hospitality, for example.
It's not really worth the TRD chasing people who are dodging the 5% tax rate on the next tranche of income because it would cost more than it produces in revenues.
It's logical to suppose that, after wealthy tax- dodging Thais, relatively high income groups like expats will be next on the TRD's list for investigation.