Bro you're making a big deal out of nothing. TM6 is not a form anyone cares about. Just get a new one at the airport when you leave. It's fast and easier to get the TM6 AND the reentry permit at the airport vs immigration in country.
If the only thing you need the Tm6 for is a reentry permit, I'd just wait and do it at the airport. Getting a new tm6 in country is a pain. At the airport you just grab one and fill it out.
You can buy a reentry permit at the reentry desk before you go through immigration at BKK. It's open