What is the best visa option for teaching English in Thailand, especially regarding onward travel requirements?

6 in one, half dozen in the other. It's not really that important, UNLESS they switch visa exempt back to 30 days. If that happens, the only way you can get 90 days is if you get the tourist visa.

As far as avoiding problems, you can be asked for proof of onward travel for both ways, since it's a requirement to enter as a tourist. The tourist visa would mostly prevent it from the airline, but immigration could still ask. If you want something to show, then purchase an onward ticket (can be found very cheap just by searching google for onward ticket) or you can purchase a bus or train ticket that some airlines might accept.

If visa exempt is still at 60 days when you plan to come, I would just do that and not even bother with the visa. Spend the money you would have spent on the visa to get an onward ticket that will take less time to get than applying for a visa.