You should bring the account back up to 800k 2 months before the next extension EVEN if you are switching to income method.
By doing this you guarantee that you will get an extension IF for some reason they deny the income method.
Any sort of mistake, or what can be interpreted as a mistake could cause you to start all over again.
For the sake of a couple more months this is the safest practice.
So the answer is are you cautious or are you certain you fully are qualifying for the income method?
I've seen extensions denied because of the money not transferred close to the same date every month and some that dropped just a few baht under because of exchange or fees.
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.