As long as the destination on the ticket is in another country, not just to a border town in Thailand.

@Alec ***

you’ve crossed there before. It won’t be any different this time.

Mae Sai immigration will want your passport. That’s it.

The airline aren’t interested in the possibility of you getting an extension. They only care about their responsibility to fly you back at their expense if you’re refused entry travelling with no visa. They will want to see proof of travel out of Thailand within 30 days.

If you're flying in with no visa the airline may want to see proof (not you saying "I'll cycle out of the country") that you will leave Thailand within 30 days of arrival before you check in for your flight.

This is very true. We had also stayed in a hotel for a few days in Bangkok before we went home but that only came up at the immigration office when we did the TM30 up in Chiang Rai. As you say, if I’d stayed at hotels the whole time, as many people do, then there would be nothing in my passport.

I would suggest that, if it’s a relatively easy trip to the local immigration office, that you do have your girlfriend register a TM30 when you get there. I’ve been travelling to Chiang Rai regularly for the last ten years or so and previously never did a TM30. However, just last month my wife did register one at our local immigration office and, on the bus to the airport on my way back to Bangkok, it was checked by the police at a checkpoint. I’m not sure what would’ve happened if I hadn’t had one, but on all previous trips a quick flash of the ID page of the passport was enough. This time it got a thorough inspection. TM6 and TM30 both inspected.

Yeah, I reckon so. They want to see that you've had the money there for six months. That will show them that.

Print it out then go to a branch and get it stamped.

You can indeed, yes. You should however be aware that staying in Thailand long term with either visa exempt entries, or tourist visas, is not going to be an option. At some point you will be questioned at a border and eventually denied entry.