You just need to be back in Thailand on or before the last day of your current extension. Hopefully, several days before to have enough time to prepare necessary documents. There is no rule on how long you can be out of Thailand with a valid extension.
Theoretically, you can re-enter Thailand again on the last day (early enough) with a valid re-entry permit then go to the local immigration office straight to apply for a new extension within the working hours or queueing time if you have everything you need to apply.
It's your decision when you want to apply for a new passport.
You can apply for the next extension with the current passport. But when a passport is valid for less than 1 year, your next extension is until your passport validity, so the next extension will be until 12 May 2026 (and the same date for the following yearly extensions).
If you get a new passport, you can apply for it as usual.
Usually, we tell people to renew their passports before the next application at the immigration office. But seeing the difference in extension is just a half month, it's totally up to you.
When you get a new passport, soon or later you have to move stamps from your old one to your new passport, at least before travelling or the next immigration-related application at the local immigration office.
Chaengwattana is rather strict with the under-consideration period. You cannot skip or change the date on your own. What you can do is contact them directly in advance to ask if you can go another day.
If they say, "no, you should go to the office on the day on your stamp", you must go on that day or you may risk losing your extension and doing the process again from scratch.
Sadly, as you are an American passport holder, you cannot use the monthly income method for the first year.
Methods to meet the financial proof for an initial Non-O based on retirement for 90 days and/or following a 1-year extension based on retirement are:
A. monthly income of more than 65k baht, with a letter of guarantee from your Embassy in Thailand
B. 800k baht in a Thai bank account
C. combination of A and B total 800K baht
The monthly international transfers method is only acceptable for the second or later year's extension.
Unfortunately, the US embassy in Thailand no longer issues an income affidavit.
So if you want to apply for a visa and 1-year extension, you need to have:
1. 800k baht in your Thai bank account for in-country visa,
2. then 800K baht in your Thai bank account for 2 calendar months when you apply for a one-year extension
3. then keep 800K baht for 3 months after an extension is granted, and keep 400K baht for the rest of the year.
You have to check the local immigration first if they accept "partially banked money + international transfers for 12 times" for a 1-year extension.
If they do, you can start monthly transfers as soon as you get an account, then use those for the second 1-year extension and after.
It could be problematic if you apply for the first 1-year extension at a different office from the one you apply for an initial in-country Non-O visa. Plan accordingly.
Q. Wondering how long it takes to get 30 more days?
-> Depends on the office and the day you go. It's supposed to be an on-spot issuance. Once you submit the documents, it will take 30 minutes or so.
Q. Do you still apply at immigration and pay 2k baht?
-> Yes, you must go to the local immigration office that serves the address on your latest TM.30. You will go to the IT Square Laksi Plaza office if in Bangkok, unless you bought an actual tourist visa before you flew into Thailand from the Thai embassy.
Unless they gave you a paper of notification, you count 90 days from your last entry date. That's your next 90-day report due.
You submit a 90-day report to the office that serves your latest valid TM.30 address. When you move, submit TM.30 to the Chonburi immigration office, then it's the office where you do your 90-day report. Make sure Chonburi don't do TM.28 to move properly.
If you mean you already applied for a 1-year extension and you are on the under-consideration period now, then go to the Chaengwattana office to get the stamp for the rest of the 1-year extension, you don't need anything other than your passport. But bringing the updated bank passbook is not a bad idea.
If you mean you are applying for a one-year extension, you need a bank letter and a transaction report up to 7 days old. You don't have to get new ones if it's issued within 7 days. You also need an updated bank passbook, original.
You put a small amount of money into your account and then update the bank passbook on the day you go to the CW office so that the officer can see your funds are in your Thai Bank account on that day.