it's 2 months prior to the date of applying for extension. Just don't mess up by a day or two. Some office also require you to show an updated bank book with the 400k when coming back getting your stamp after the under considérations Tamp.
What are you talking about 6 months of transactions ? Did your immigration office asked you to prove how you sustained yourself in Thailand ?
10min is wonderful. The key is to gather information and be prepared. But it's still risky nonetheless to apply on the last day. Others are right. Many things can happen like : no more queue numbers for today 🤣(happened to me at Lak.Si) or an officer wanting some additional document or rejecting one you have. It's not unheard of, rules are not set in stone here. Great it did work out smooth for you 👍🏼