Have you considered getting a non-O based on being over 50 before coming to Thailand instead? It's probably the easiest visa to get and means you can open a bank account when you get here, so that you can extend your stay for a year. You can change it to a marriage extension the second year. Many people never bother changing to a marriage visa/extension (myself included) as it's a more complicated process.
is it an electronic copy that they send online or does it go to your branch David? Also, do immigration offices readily accept this version of statement? Many thanks.
from what you are stating here, it seems like the non-OA visa is the one for you ( multiple entry, no requirement to bank money in Thailand and for between 1 and 2 years). It lasts for one year, but I read that if you work it correctly you can get approximately 2 years from this visa, by arranging a second years insurance and flying in and out of Thailand at the end of the first year. The question about when to do it, i guess depends on the visa you choose, but I believe the non O and non OA are valid for entry for 3 months, so you probably want to apply in about January 2026. Brandon has given you excellent advice on your other points.
The extension is granted on the same day you apply. You can apply for the extension 30 days before your non-O ends, providing your money has been in the bank for 2 months and the days are added to your extension (if that helps with your timings).
Get your non-O retirement visas online from your Thai embassy before coming to Thailand. It's virtually impossible to open a bank account in Thailand with a tourist visa, so you need the non-O first, you can then extend your stay. It used to be possible with a tourist visa, but the banks became very strict earlier this year.
Okay Colin, it's more than the extra things required to get the marriage visa. You can get the non-O retirement visa online before coming to Thailand. This enables you to open a bank account to extend your stay for 12 months. You can then concentrate on getting married. The following year you can extend your stay based on marriage. It's a much easier process, but it's up to you if you want to make it difficult for yourself. Good luck with it all.