What should I do if my bank balance is just under 400,000 baht for my Non-O married visa?

This happened to me last year, they just extended my current visa another 30 days to meet the requirements of 400K. Went back and all good.

Just have a joint account and leave the money in there all the time

FAFO is all I say. Visit Immigration and see what they say. You still have the option to apply for the 60-days extension based on family visit. This would give you more money maturing time

I've had the same a few years back. No problem. Just remain friendly and explain. I got a fine, that's all, yearly extension was ok no problem.

If your wife has money in her account you can use both accounts. As long as you had 400k total in both accounts that should be fine.

If it was me I would go the the immigration office well in advance like now and ask them for advice. I know you are below the limit but they they used to take all Thai personal accounts together so if you have another account you are probably still OK. In addition, I'm on a retirement visa and I normally renew 1 month before and it's always dated from the expiry of the current stamp. I understand that if you renew a retirement within 40 days of the expiry date it's run from the expiry date, anything before that and it's run from the date of renewal, I don't know anything about a marriage visa though. If you go to immigration well in advance and present all the right documentation and they say it's OK then you might be able to renew it then and there, OK losing a month or so but you would leave with a new stamp. The only problem with this idea is that a marriage visa needs approval from higher up so might be OK at the local office but rejected anyway but again should give time to sort something out if you do it before the expiry date.

Not all offices will grant an extension of 60 days to visit family, so go and ask.

Or u just can pay for service

Apply for the 60 day visit family extension if you can get it, it's normally only allowed once. Otherwise you will have to exit Thailand and start over. Immigration are pretty strict on the financials

It is ultimately at the discretion of the immigration officer, and a one year extension of stay may well be refused in the current circumstances. Applying first for a 60 day family extension and allowing the funds to complete the required seasoning period is likely the more prudent approach.

Sounds like your only option. Why put yourself through the stress, keep a buffer of a few thousand baht in there at least.