I'm also due at the same time as OP, I've never done an extension before and already getting anxious about it given the problems I ran into the first time around.
Is getting a marriage extension (actually mine is based on Thai child, but I assume they are much the same thing) really so simple it could be done a few days in advance of expiry? I'm planning on going next week (well in advance by the looks of it).
I've basically just reprinted and updated everything I did for the first 1 year visa, I hope this suffices.