, You can. But if you want to make things easier, keep a copy of everything, anything that your spouse needs for the cancellation of his extension, just in case. Such as the termination letter, cancellation receipt of work permit, cancellation stamp on his passport, etc. His cancellation stamp might be sufficient, but it's not a bad idea to keep copies for your turn.
Q. I think it was helpful that the re-entry permit had been issued by CW,
-> Yes, CW wouldn't move a re-entry permit if it's issued at the airport. In that case, you need to go to the airport that issued a re-entry permit to move.
Q. When would I need to have the money in the bank
-> You must complete two months as of the day you apply for your next extension, not two months until the due date. You have to put 800K 25 July at the latest. But in this case, you have only one day to prepare bank documents and submit the application. Put your balance to 800k baht as early as possible.
Q. would I have to have it shown as being from a foreign source?
-> 800K baht doesn't have to be transferred internationally for an extension application. But some offices MIGHT want to see the proof of source. Check with your local immigration office.
Q. I have noticed in 2020 the extension was to 24th Sept, and in 2021 and since has been stamped 26 Sept
-> Cannot say anything for sure without stamps. But a big possibility is that the Covid amnesty might affect your stamps during those years.