What are the requirements for renewing a retirement visa in Thailand, especially regarding bank funds and documentation?

I messaged the O/P and at this time I think they got about as much info as they can from the group and comments. I'll close commenting for now

It appears the O/P did NOT follow the seasoning requirements for the extension they're on < letting the balance go below 800K during the 3 months after the extension was granted.

Now at this time they're unlikely to have any issue and BUT when they go back to the immigration office next year to apply for a new yearly extension of stay believe me Immigrations WILL see that the seasoning terms were not followed.

AND

As was pointed out, 'technically' you void your extension the day you fail to follow the seasoning (meaning you were in theory on overstay since that date) <- this is IF the immigration office wants to take a hard line stance.

Sadly for the O/P there is no way to 'restart' inside the country.

You'd have to leave the country, (getting off the current extension), then get a new 90 day Non-O visa at a thai consulate in a nearby country, come back and 2 months later apply for the yearly extension

OR

exit (getting off the current extension) /re-enter the country on a 60 day visa exempt entry ,then apply first for the 90 day Non-O visa and after that the year extension

I would say you run almost a 100% chance that immigrations will notice the shortfall in your balance <- that's why you have to get a year detail transaction report, so they can see the balance on the account every transaction during the year to verify you followed the seasoning.

I would definitely be planning you're going to have to start the whole process all over from scratch

Better keep 3 months before your current one expire and keep that Months after getting renewed visa. But make sure it should not fall below 400k anytime during 1year

You must hold 800K baht funds in your Thai bank account for three months after you get the first extension, then keep no less than 400K baht for the rest of the year to keep being eligible for the retirement extension. And you must have 800K baht again for the full two months when you submit your application next year.

Required documents are the same, but you may want to make all new ones.

You may need a renewed lease agreement if you have one.

The Chiang Mai office accepts your extension application when your stamp is remaining 45 days or less. You are free to apply for an extension anytime within this period.