Need advice from the experts please.

My current marriage extension runs out on 17th September but I can't get an online appointment at CM immigration until the day after. What's best to do; go in the day before and suffer waiting most of the day or take the appointment one day over and pay the fine?

You have 30 days before it expires in which to apply.

There must be an appointment within those 30 days.

You have 30 days before it expires in which to apply.

There must be an appointment within those 30 days.

Sadly there isn't a slot available between now and expiry date !

Hopefully someone from Ching Mai will chip in.

The choice is yours, both are possible.

But no overstay is always the best and I would go earlier than the last day, in case you need to get some extra documents.

The extension starts on the last day of your current permission to stay, so no days are lost by doing it early.

Don't wait until the last day - go a week or two early and arrive early to obtain service that day - in case too many that day you still have time to return later that way. I would also keep checking the appointments as it could change if you are lucky.