Apply for the Non Immigrant O visa based on being married before he makes his way into Thailand and get 90 days of stay, which give enough time to prepare the finances to apply later for the 1 year Exension of Stay based on being married.
Or
Get into Thailand using the 30 day Visa Exempt Entry, apply for Extensions of Stay based on tourism and visiting wife to get the time to prepare the financial requirements for the Non Immigrant O visa based on marriage.
As the advice is correct but it is a strange way and more expensive.
OR apply for the Single Entry Non Immigrant O visa based on marriage before you make your way into Thailand (2500 Thb) and apply in Thailand for the 1 year Extension of Stay (1.900 thb).
OR
Go to Thailand and get the free 30 day Visa Exempt Entry,
Go Immigration to apply for a 30 day Extension of Stay (1.900 Thb)
Go Immigration to apply for a 60 day Extension of Stay (1.900 Thb)
Go Immigration to apply for the Non Immigrant O visa based on marriage (2.000 Thb)
Go Immigration to apply for the 1 year Extension of Stay (1.900 Thb)
The same insurance as you had when you applied for this visa and used it to get 1 year of entry in Thailand.
The new and higher insurance policy is at the moment only introduced at some Thai Embassies abroad and the idea is that from 1 October 2022 it will be introduced at the Immigration Offices but that is not official as we wait for the announcements.
Indeed no "hard and fast" rule, but people are not always capable to convert their Visa Exempt Entry or Tourist Visa into a Non Immigrant Visa. Sometimes Immigration has more requirements that an Embassy or the local office does not have the credentials to perform conversions.
As this Extension of Stay is only for children up to 20 years of age, you should be aware that there is a possibility that Immigration only allows her to stay until her birthday in Aug and you have to seek an alternative visa or Extension of Stay for her before her birthday.