If you come to Thailand 1-Jan you need to plan which neighbouring country you will exit to for your DTV application before 1-Apr as you cannot still be in Thailand then without a proper visa.
For an in country Non-O Visa (90 days) 400k in a Thai bank account on the day you apply then again 400k for 2 calendar months before you apply for the 12 month extension
After you've taken your first 12 month extension based on retirement you, can when you apply for a new extension, change the reason to based on marriage by meeting the requirements