Do you have a monthly income/pension of minimum equal to 65K baht (€1755)? If so your embassy/consulate in Thailand offers to provide you an affidavit letter you can use at the immigration and apply for a Non O based on retirement in country with monthly income. Otherwise there’s no legal way around the financial requirement for long stay in Thailand.
If you really had a 15 days Visa on Arrival you only would get 7 days which in fact is an extension denied stamp. However If you entered on a 60 days visa exemption you should be able to apply for one 30 days tourist extension at your local immigration. You don’t need a TM.6 and TDAC is only for arrival.
Bring a receipt of your TM.30 address registration, a pass photo, copy of ID-page and last stamp in your passport plus 1900 baht in cash.
easy and best option is to apply for the initial 90 days Non O visa online at home before you come and do yearly extensions of stay at the immigration. Price for the visa is normally around 80 USD. One year extension of stay in Thailand is 1900 baht. No need to use an agent if you fulfil the requirements.
Agents are probably around 25-35K baht first year and 15K baht for the second year if you fulfil formal requirements. If you need agents to bypass any requirements -which formally gives you a void stay- you’ll have to pay more.
Both the regular 90 days Non O visa and the one year Non O-A visa require min 50 years of age and monthly income min equal to 65K baht or funds equal to 800K baht.
The Non O-A visa also requires a mandatory health insurance, health certificate and a police check up.
The regular Non O visa gives you up to 90 days in Thailand before you’ll need to apply for yearly extensions of stay in Thailand more or less on the same conditions as the initial 90 days Non O visa.
The one year Non O-A visa gives you up to two years in Thailand if you update your health insurance also for the second year.
After this you’ll need to apply for yearly extensions of stay as for a regular Non O visa BUT you also maintain the mandatory Thai health insurance requirement for all future.
You can apply for a Non O visa based on retirement in country on a TM.87 for visa exemption. You’ll need minimum 15-21 days left on your current stamp. The requirements are the same for both entries.
If you’re from one of the 93 nations entitled to visa exemption you’ll will as a general rule receive a 60 days free stamp upon arrival as you did last month until the day the immigration officer you’re standing in front of no longer consider you as a genuine tourist to Thailand.