The Non Imm O is the marriage visa, you can apply for the 1year extention of stay based on marriage when you have 30 days left on the visa. You'll need 400k Thai Baht seasoned for 2 months in your Thai bank account prior to your application
90 days visa and 60 days extention is the max without marriage or court approval. You are correct that you used to be able to get a multi-entry Non Imm O but that hasn't been issued for a half Thai child for quite a few years now.