What is the best visa option for Australians planning a 14-week stay in Thailand for volunteer teaching?

Volunteer visa is probably the best suggestion. However, if you don't want to deal with the bureaucracy the tourist visa is right for you. You can do back-to-back or visa runs.

The retirement visa isn't a bad option but it requires more paperwork. Really, its the 14 week part, 90 days + 2 week part, that complicates your situation.

If you speak Thai and already volunteer teach than you are already familiar with how the rules are flexible. To that end, I would just do a visa run. Your travel history to Thailand would suggest your are frequent visitors and not permanent visitors. It's unlikely immigration or the police are going to have an issue with you being decent people.

You will have to apply for a volunteer visa and that make take some time.

You cannot volunteer as that is considered work and you need both a work eligible visa as well as a work permit to do so.

If you're just coming to visit then you can apply for a tourist visa before you come. This will give you 60 days on arrival and you can apply for a 30 day extension.

That will be about 12 weeks and then you do a border bounce by leaving and returning to Thailand and get a 30 day visa exempt stamp for the rest of your time.