What alternatives are available for extending my Non-B visa in Thailand without tax documentation?

Make a visit to the Tax office, declare your income, pay income tax for three months and problem solved.

I described a Business B Visa which you get by becoming employed by a Thai Registered Company or start such a company of your own. A twelve month visa is not available to someone under age 50 unless it is a B Visa or an O visa based on Marriage or caring for dependent Thai children. There is an Ed Education Visa but that now is a series of 90 day visas that must be renewed.

Your best shot is the METV Multi Entry Tourist Visa that has a term of 6 months with a series of 60 day stays which can be renewed with a Border Bounce and Extended by 30 days at a Thai immigration office. You can get the METV to last 8 to 9 months by timing a border bounce just a few days before the 6 month expiry - getting another 2 months which can be Extended 30 days. You get the METV at the Thai Embassy / Consulate in your home country. Requirements are proof of having a job in your home country /outside of Thailand, or business ownership, plus have a bank account with the equivalent of $7,000 in it. Plus, some other minor requirements.

I'm coming to Thailand next year, I'm 42 and retired, will I be able to get 12 month visa?

12 month multiple entry Non imm visa in Penang, Malaysia. Must Exit and Reenter Thailand (Border Run / Border Bounce every 90 days. The 90 day Border Bounces require No documentation, no traveling to the interior of the neighbor country. Not as good as a one year Extension but better by far than getting a new non imm O every 90 days.

the only other way to get an extension of stay for 1 year is either marriage, have children/family or retirement 50 and older. normally people can get the extension based on work but since you are lacking the Tax documentation then there isn't any other way around that