However, my current PC policy is in the database for travel purposes but I guess that doesn't count or it's just not possible to do it this way this time.
Interesting. I have had the same PC policy for a couple of years. But since I haven't used it to extend yet. They, I guess, don't know about it. So yes, maybe next year I won't loose time when I try to extend early.
Get a 1 year statement from your bank. Bring your bankbook. They may want to examine it. If they want copies then they will tell you. Since you have your bankbook with you it should not be a problem to get copies same day.
Your multiple entry probably expires when your visa or extension expires. That should not be a problem. You confirm with your immigration office that you can apply for an extension of your OA visa by reason of marriage and not be subject to the health insurance requirement.
I would like to confirm the answer to this. I have insurance but that would be plan B.
You will need the Thai insurance if you apply for a 1 year extension when you are inside Thailand. You will need the insurance to apply for a new O-A visa in your home country.