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.