What should I do if my visa runs out before I can leave Thailand?

Call an agent.

So you've been in Thailand for 18 months as a tourist, and you want to bounce to get another month? All you can do is try. I think you'll encounter some resistance as immigration may look on it as simply taking the piss. Make sure you've got the three proofs especially that outbound ticket.

If questioned by immigration on arrival (very likely to happen on your case as you’ve had covid extensions) you need to provide 3 proofs for entry into Thailand.

1. 20,000 baht cash or equivalent in a major currency. It needs to be in cash. A bank statement, credit card or phone app showing you have funds won’t suffice.

2. Accommodation booked for at least the first few days. If you’re staying with family or friends then have them write an invitation letter for you to stay at their place.

3. A ticket out of Thailand booked within the number of days your entry allows, regardless that you can get an in country extension. 30 (will be 45) for Exempt, 60 for Tourist Visa (although the likelihood on being asked if you have a actual tourist visa is lessened). Bus train or plane. The two former must be an international ticket out, not just to a border town where you could walk across a border.

These proofs won’t guarantee you entry as that’s at the discretion of the immigration officer, but they’re the three things they’ll generally use as a reason to deny entry.

Times up

Almost 100% chance you'll have problems. You're going to try and leave Thailand and re-enter as a tourist when it's clear you've been living in Thailand for over a year and you are not a tourist. Highly likely they either deny entry or grill you and ask a lot of questions and want to see a lot of documents.