What is the easiest visa option for Canadians to spend 5 to 6 months in Thailand?

Multiple Entry visa if you bounce right you can get almost 9 months out of it

It might be a little overkill, but for the absolute simplest time for you which will only involve you having to leave Thailand one time during your 6 months, you should apply for the Multiple Entry Tourist Visa.

This will give you 60 days in Thailand every time you enter during the 6 months of the visa validity.

When you arrive the first time you'll get a 60 day stamp and you can apply for the 30 day extension. At the end of your 90 days do a border bounce either to a land border or fly to a neighboring country and back (even in the same day if you want, as long as you cross into that country through their passport control) and you'll be stamped in for another 60 days which you can once again extend for 30 days.