I have not activated it and make a point while in transit of switching my roaming to the correct sim ready for arrival. Although roaming is off I can still receive Thai sms ro my True inbox as it connects to the network but without data.
Why doesn't your Thai sim work in Canada? It should connect to a local network just as mine does in the UK (I have dozens of TrueMove and others sms in my inbox to prove it).
I do something similar with stays in Thailand of just under 90 days and stays "at home" of just over 90 days. I do this with a Non-O marriage Visa as this does not require proof of onward travel to fly/enter and does not require a visit to Immigration for an extension. This is the most effective use of my resources and costs in my opinion.
If you've been out of the country you usually have to file a new TM30. The "bank letter" is a standard letter that your bank will know when you say you need it for a retirement visa extension. It might take up to a week to obtain.