If you are aged over 50 then you can apply for a 90 day Non-O visa in your home country. If your embassy won't provide an income certificate to you you can obtain a 12 month extension by putting a minimum of 800k baht in a Thai bank account.
You apply within 3 months of your proposed date of travel. If you apply earlier they won't process it so make sure the date you supply fits. Once you have the visa you can anter as often or as little as you wish
you will struggle to open a bank account on a tourist visa or visa exempt entry these days. It's probably easier to leave to a nearby country and apply for the 90 day Non-O there using funds in your home country. Once you have it return to Thailand, go to immigration and get a residence certificate, get a mobile SIM registered against your passport and then visit a bank to open an account and deposit the funds required for the 12 month extension of stay
You can convert the DTV to another visa type at your local immigration office as long as you fulfill all the criteria for the visa you want to convert to