Do I need to show proof of onward travel when entering Thailand with a 60-day tourist visa and planning to extend it for 30 days?

Super excited for my first trip to thailand and SE Asia! I booked roundtrip flights from Bangkok, leaving from the US in september and returning in February. I'm going to do a 60 day Thai tourist visa, and then would like to extend for 30 more days, before moving on to Cambodia and then Vietnam.

My question, do I need to show proof of onward travel from after 60 days or 90 days, since i will be extending once in country? upon entering, will I have to show a flight out after 60 days even if i plan to extend for another 30? Been trying to find a clear answer to this, but no luck yet. thanks!