As you're on an Extension of Stay based on work, this will end once you stop work and your WP is cancelled. You will have 7 days to leave. You should be able to change the reason for your extension to marriage, assuming you're married to a Thai and meet the financial conditions. As Billy Brown says, go and talk to your local Immigration office.

Most hospitals will do it. I've used Bumrungrad (thorough but expensive), Phaya Tai 2 and Sukhumvit.

Maybe it depends on the Immigration Officer, but I've only ever had one TM30 filed at CW, despite having numerous trips within Thailand and annual extensions since then.

@David ********

Did the OP have to do a written test for his renewal or is it only for special cases?

Did you ask why the pool was closed? Maybe it was for essential maintenance? No Covid related reason for closing pools in Bangkok any more.

There isn't supposed to be a charge for transferring stamps. Which Office was this?

@Danny *******

No idea. You can choose either from Boots. The saliva test is recommended for kids

@Danny *******

Do the saliva one if your nose is sensitive

@Ray ********

You'll need to present the old passport at BKK airport when you arrive so that the IO sees the Extension stamp and Re-entry permit. He will then refer to this when he stamps your new passport. Take both passports when you go to your Immigration Office to get the Extension transferred. I've done it before, and it was straightforward. They don't transfer the Re-entry permit; you have to do this at a different desk (free).

@Ray ********

Actually the letter is only required if you get a new passport in Thailand. If you get a new passport in the UK it will be stamped when you arrive in Thailand, so won't be blank when you take it to Immigration to get your Extension transferred.