Can Canadian citizens extend their visa exempt stamp in Thailand, or do they need to do a border run?

As a Canadian passport holder you do not get a visa on arrival, you qualify for a 30 day visa exempt stamp upon arrival in Thailand and yes you can extend this at your local immigration office one time only for another 30 days for 1,900 baht, giving you a total of 60 days in Thailand. Check with your local immigration office as some offices require you to have a up to date TM30 before you can get an extension.

You do not have any visa. You have visa exempt (exempt from needing a visa, no visa at all).

You can apply one time for a 30-day extension.

If you're going to another border bounce after this, that's probably going to be the last time you enter Thailand without a long term visa. You can only do 2 land border bounces without a visa per year, and with an ED visa in your history and 120 days of visa exempt after that, I'd say you've got about a 75% chance of being denied entry at an airport without a long term visa.

You got 30 days visa exempt stamp on arrival. Yes that can be extended for another 30 days at immigration office near where your TM30 is registered