Your home country driving license together with an IDP allows you to drive for 3 months on each entry into the Kingdom. If you need longer you can apply for a temporary license which is valid for two years and can be got with a visa exempt entry or a tourist visa. If you are on a non O based on marriage or retirement you can apply for the 5 year license after first getting the 2 year temporary license.
The Thai interest paid on your 800k held in a Thai bank account will most certainly attract withholding tax , resident or not. Now if the original 800k transfer is taxable depends on your tax residency in year of transfer, the source of the 800k and any relevant DTA in place. So the answer is, it depends.
suggest you do some reading on Thai tax law and pay attention to the terminology. Only “assessable” income is subject to personal income tax. Everyone with “assessable” income above minimum thresholds is required to file a tax return.
After 180 days in the Kingdom within a calendar year you become subject to Thai tax law. You will have a Thai tax liability only if you have assessable income. Whether you will actually pay any tax depends on your personal situation.