I got my bank letter on a Friday and went to Immigration on a Monday. I had a small problem that day. They told me that they would keep my paperwork and she reminded me that the bank letter would only be good until Friday. I returned on Wednesday and completed the extension application. At least on that occasion they were OK with a week old bank letter. My philosophy - don’t wait for the last day.
I can report that there was no line to file a 90 day report at MTT last Thursday. I arrived just after 9am and literally walked through the process and was out 10 minutes later.
There may be more requirements for an OA so you need to look at the details for both to decide but the logistics of an OA may be preferable. Good luck.
If you get an OA you can use money in you home country bank accounts. Once you get the OA you just arrive and there is nothing else to do. If you come on a non-O then then while you are here you will have to open a Thai bank account (if you don’t already have one), transfer the money, and then go to Immigration to apply for your year extension. Question: do you want to be occupied doing that after you arrive?
If this is your first extension of your OA visa then you have to have your Thai health insurance active and in the database before you apply. Your extension will only go until the end of your insurance. If you apply a few weeks early then you will lose a those weeks from your 1 year extension. This only happens the first year. After that you will always have insurance when you apply.