How soon can I request a 30-day extension on my 60-day tourist visa in Thailand and change my return flight date?

Hello everyone, I hope you’re all well. So I finally got my SETV approved from the Thai Embassy in Paris (it took 5 weeks and I used the services of an agency- Rapidevisa). I’ll arrive in Bangkok on December 17. My return flight is booked for Jan 13, however I plan on staying longer; between extensions and leaving the country once, I plan on returning home at the end of April. Two questions: 1. How soon can I request the 30 day extension on my current 60 day tourist visa? and 2. How soon can I rebook my final return flight on April 30? If need be, in the interim, I’ll book a cheap onward flight when requesting other extensions and/or visa exemptions. The thing is, I don’t want to have to rebook my final return flight more than once and, obviously, I need to change the return date before January 13. Thanks for any suggestions you can give me. You guys have been immensely helpful.