If your country has a double taxation agreement with Thailand you won’t pay tax twice but you will pay the higher rate. Some DTAs exempt certain types of income.
180 days inside Thailand in a calendar year and you automatically fall under the jurisdiction of the Thai Revenue Department. For you to pay tax you must first have assessable income, either domestic or foreign remitted. The RD doesn’t track anything (putting CRS aside) the law states it is up to you to self declare your income to them in your tax filing. No income (living off savings for example) no need to file taxes.
All easily done without using an agent. Apply for the Non O visa from your home country using home country funds. Enter on that visa, open bank account, deposit funds within 30 days of arrival, go to immigration, get list of extension requirements. Compile documents and apply for extension, 1900 baht.
this is wrong. There is no 90 day limit under Thai law or in the Conventions. They are repeating an urban myth. You need to actually read the law, section 42 of the Motor Vehicle Act 1979 covers the legality of driving in Thailand, no mention of limits of an IDP.