The easiest for a year is the DTV for a family. Unless you are putting kids in school and getting guardian visa through them but that will be expensive.
Not all income is assessed. Only assessable income. You are inferring in that sentence all income will be assessed if you are a tax resident. It is not. Are you a Thai Tax Resident and have you ever submitted a tax return in Thailand?
Immigration did NOT set the rules for the DTV. It was the MFA. Immigration do have final say on right of entry though even with a visa. The MFA website clearly states that.