At the end of the 90 day non-o visa, you may be able to apply for visit Thai family 60 day extension.
This will give you time to get the funds back up to 400k.
You can do custodian visa but depending on the child's age you would need court ordered custody or if they are older a letter from the ampur with at least 50% custody agreed upon.
You would need to speak to your local office for clarification.
But either of these scenarios can work if you apply for the 60 day visit Thai family extension and you won't need to start again.
unfortunately in the past it was relatively easy to open an account as a tourist. It is not any longer. This in itself is not an immigration problem but a banking issue. The ability to change status to non-o is there IF you qualify. If you do not qualify it must be done outside of Thailand as designed.
IF you have a 90 day non-o, your local office may offer the 60 day visit Thai family extension allowing you more time before applying for the 1 year. Ask at your local office.
First thing to is contact the US thai embassy you will use and ask this question:
"My husband will apply for a 90 day non-o visa based on retirement.
I would like to apply for a 90 day non-o visa as dependant spouse.
Is this possible?"
There are still a few embassies that will issue a retirement and dependant visa.
Then you could both enter with the visa and extend 1yr in Thailand once he has money in the bank.
Having the visas makes opening a bank account much easier.
You would be his dependant and not require to wait till you are 50.
IF YOU choose to:
You husband could get an 1yr OA VISA , thats requires health Insurance perpetually, and you could apply for a 90 day non-o dependant on his visa and then extend for 1yr in Thailand.
There are a couple ways to make it easier without needing 1.6mil baht in a bank. You would only need 800k in both these scenarios.
In both cases getting your e-visa first is the best course.
Getting the bank accounts in Thailand right now is very problematic without at least the 90 day visa.