Depending on the immigration office you can apply for your 1 year extension 30 or 45 days before the extension expires. You do have to be here in that time window so pick your 1st extension month to be convenient for you to be in Thailand.
The OA and O visas can be extended 1 year at a time when the visa expires. You can get 1-2 years from an OA and 90 days from an O visa. What you get with an OA is that you can qualify with money in your home country bank and you get a one year visa. After the 1-2 years on the OA you can extend 1 year at a time. The OA requires insurance which may or may not be a good thing depending on your insurance circumstances. The O is only 90 days but it doesn’t matter much because what you are really trying to do is get to 1 year extensions. The benefit of an O is that it doesn’t require insurance (you can do that on your own separate from the Visa). For both the OA and O visa, for the 1 year extensions you must meet the financial requirements using money in a Thai bank. For the extensions you will need a Thai bank account which appears to be somewhat difficult these days.
You can do all this yourself or hire an agent. I always did it myself. There are pros & cons to either method.
The taxable income is money transferred into Thailand. I’m not aware of any clarity regarding the tax obligations of individual expats. It is definitely a thing for businesses earning money overseas and bringing that money back into Thailand.
Do you mean LTR? The Thailand Board of Investment is telling LTR holders over and over again that they are not subject to tax on any income brought into Thailand. That was a feature of the LTR vise before the new tax rule was proposed. Never say never though . . .
Only during the first year that the OA visa is valid. You can’t do this indefinitely. The most you will get is almost two years on the original visa. After that you have to extend the OA inside Thailand or go back to your home country and start again with the OA or some other visa.