@Christian ********
you have two options:
ONE:
***exit Thailand, re-enter visa-exempt, apply for the "change of visa type" from the visa-exempt entry to the initial 90-days Non-Imm-O Family Visa on Immigration. The fee is 2000 Baht. The visa will get issued “under consideration” for 14 days. That’s why you still need 15 days on your entry stamp if you want to apply for the “change”
You need to show a minimum of 400,000 THB in your Thai bank account when you apply for the “change”
And you need to show a minimum of 400,000 THB having seasoned in your Thai bank account for 2 months to be able to apply for the “1-year Extension of Stay Permit”
TWO:
*** exit Thailand, visit any country anywhere in the World including your home country. Apply for the 90-days single entry Non-Imm-O Family Visa through the e-visa online system of the Thai embassy in this country. For this application you must prove that you physically are inside this country!
Wait in this country until the visa has been approved and issued to your email-box as a .pdf document. Print it and re-enter Thailand. You will get stamped in for 90 days
In most countries, you need to show a minimum of 400,000 THB in your Thai bank account for the last three months, when you apply for the 90-days Non-Imm-O family Visa
And after having entered Thailand on the Non-O visa, you need to show a minimum of 400,000 THB having seasoned in your Thai bank account for 2 months to be able to apply for the “1-year Extension of Stay Permit” on Immigration