What type of visa can I convert my Single Entry Tourist Visa (SETV) to for an extension in Thailand?

I am 70 yrs of age and invested heavily in my Homestay in Chiang Rai. The Homestay did realise the investment I would expect. Today my Thai wife goes back to Oz to take up a position of employment. And also to work with her future In laws. Her son claims benifits. This is relative to my question. He recieves student support also from Centrelink. And I just repaid $18000 debt she accrued - Unknowingly to me.

I mentioned in the past. I wish to convert a SETV to a - what type of visa - I have a stat dec from the Oz embassy to secure the extension of 12 months . I thank you for your advice. My SETV is valid until March 24. I need the extension to foreclose my relationship and conclude my residence in Thailand.