ALL offices will let you apply for a new yearly extension when you have 30 days or less left on your stamp (and as people pointed out Bangkok, Chiang Mai and others allow you to do it when you have 45 days or less left on your current stamp)
The 800K baht will have to be in the account for 2 months before you apply for the extension so if you're applying Sept 1st (a month before your current extension runs out, you need the account to have 800K baht in it by July 1st at the latest to hit the 2 month seasoning)
BUT
You definitely have time to get your new year extension AND a re-entry permit before you travel out of the country.
They shouldn't have even SOLD you the re-entry OR stamped you in on those pages really 😮
I'm saying you need to slog on over to your immigration office and see what they say when they see those are the only pages you have left 😕 They'll either do it or tell you to get a new passport
Right now for US people using the mail in renewal takes about 35-40 days to get both passports back in the mail at your address
#1 - As long as you're NOT an amercian citizen on an "old style" passport (the one without the hard data page) because the last two pages of those old passports are for endorsements (and immigrations won't stamp extensions on those pages
#2 - Apply for your extension, get it inked in, get a re-entry permit and it's valid from the date you applied for it until your extension expires next year (y) No problem using it right after you got it
This is what the US passport pages that are only for endorsements look like, the bottom image is endorsement pages
TM30 showing your registered as staying at an address served by Bangkok Immigrations, you should also do your 90 day reporting here at that address (if possible) rather than move just before your extension is due.
In Bangkok you can apply for a new extension when you have 45 days or less left on your current stamp. You need to have the year print out of the detail transaction report, the standard bank letter and update your bank book the date you apply for the extension, TM30, lease, land lord thai i/d maybe the house book copy. Remember if you used banked money method you needed to have kept the 800K baht in the account for 3 months after the extension was granted last year and then the balance couldn't go below 400K the rest of the year until you get it back to 800K baht a minimum of 2 months before you apply for the next yearly extension
This is a generic list of what's required at most offices. It is NOT office specific so I'd suggest you slog on out to Chaengwattana and get the hand out listing the requirements for a yearly extension based on retirement.