correct, pensions are assessable income unless exempted by a DTA. If exempted then no tax filing. If not exempted then just normal PIT rates, allowances and deductions apply with any withholding taxes paid in home country allowed as a tax credit against your Thai tax liability and again you are correct in this situation you would need to file assuming your remittances are above minimum thresholds.
not at all, it is very common to live long term in Thailand with no tax liability, Canadian pensioners, US citizens living of social security, UK pensioners living off government pensions etc etc.